Integrated device for temporal binning of received photons

ABSTRACT

An integrated circuit includes a photodetection region configured to receive incident photons. The photodetection region is configured to produce a plurality of charge carriers in response to the incident photons. The integrated circuit also includes at least one charge carrier storage region. The integrated circuit also includes a charge carrier segregation structure configured to selectively direct charge carriers of the plurality of charge carriers into the at least one charge carrier storage region based upon times at which the charge carriers are produced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/914,019, filed Mar. 7, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/656,139 (now U.S. Pat. No. 9,945,779), filedJul. 21, 2017, which is a continuation of U.S. patent application Ser.No. 14/821,656 (now U.S. Pat. No. 9,759,658), filed Aug. 7, 2015, whichclaims priority to U.S. Provisional Patent Application No. 62/164,506,filed May 20, 2015 and U.S. Provisional Patent Application No.62/035,377, filed Aug. 8, 2014, each of which is hereby incorporated byreference in its entirety.

This application is related to the following U.S. applications:

-   -   U.S. Provisional Patent Application 62/035,258, entitled        “INTEGRATED DEVICE WITH EXTERNAL LIGHT SOURCE FOR PROBING,        DETECTING, AND ANALYZING MOLECULES,” filed Aug. 8, 2014;    -   U.S. Provisional Patent Application 62/035,242, entitled        “OPTICAL SYSTEM AND ASSAY CHIP FOR PROBING, DETECTING AND        ANALYZING MOLECULES,” filed Aug. 8, 2014    -   U.S. Provisional Patent Application 62/164,464, entitled        “INTEGRATED DEVICE WITH EXTERNAL LIGHT SOURCE FOR PROBING,        DETECTING, AND ANALYZING MOLECULES,” filed May 20, 2015;    -   U.S. Provisional Patent Application 62/164,485, entitled “PULSED        LASER”, filed May 20, 2015;    -   U.S. Provisional Patent Application 62/164,482, entitled        “METHODS FOR NUCLEIC ACID SEQUENCING”, filed May 20, 2015;    -   U.S. Non-provisional patent application Ser. No. 14/821,686 (now        U.S. Pat. No. 9,921,157), entitled “OPTICAL SYSTEM AND ASSAY        CHIP FOR PROBING, DETECTING AND ANALYZING MOLECULES,” filed Aug.        7, 2015; and    -   U.S. Non-provisional patent application Ser. No. 14/821,688 (now        U.S. Pat. No. 9,885,657), entitled “INTEGRATED DEVICE WITH        EXTERNAL LIGHT SOURCE FOR PROBING, DETECTING, AND ANALYZING        MOLECULES,” filed Aug. 7, 2015.

Each of the above-listed related applications is hereby incorporated byreference in its entirety.

BACKGROUND

Photodetectors are used to detect light in a variety of applications.Integrated photodetectors have been developed that produce an electricalsignal indicative of the intensity of incident light. Integratedphotodetectors for imaging applications include an array of pixels todetect the intensity of light received from across a scene. Examples ofintegrated photodetectors include charge coupled devices (CCDs) andComplementary Metal Oxide Semiconductor (CMOS) image sensors.

SUMMARY

Some embodiments relate to an integrated circuit that includes aphotodetection region configured to receive incident photons, thephotodetection region being configured to produce a plurality of chargecarriers in response to the incident photons. The integrated circuitalso includes at least one charge carrier storage region. The integratedcircuit also includes a charge carrier segregation structure configuredto selectively direct charge carriers of the plurality of chargecarriers into the at least one charge carrier storage region based upontimes at which the charge carriers are produced.

Some embodiments relate to an integrated circuit that includes aphotodetection region configured to receive incident photons, thephotodetection region being configured to produce a plurality of chargecarriers in response to the incident photons. The integrated circuitalso includes at least one charge carrier storage region. The integratedcircuit also includes means for selectively directing charge carriers ofthe plurality of charge carriers into the at least one charge carrierstorage region based upon times at which the charge carriers areproduced.

Some embodiments relate to a photodetection method, comprising receivingincident photons and selectively directing charge carriers of aplurality of charge carriers produced in response to the incidentphotons into at least one charge carrier storage region based upon timesat which the charge carriers are produced.

Some embodiments relate to a computer readable storage medium havingstored thereon instructions, which when executed by a processor, performa photodetection method. The method includes controlling a chargecarrier segregation structure to selectively direct charge carriers of aplurality of charge carriers produced in response to incident photonsinto at least one charge carrier storage region based upon times atwhich the charge carriers are produced.

Some embodiments relate to a method of forming an integrated circuit.The method includes forming a charge carrier confinement regioncomprising a photodetection region and a charge carrier travel region.The photodetection region is configured to produce a plurality of chargecarriers in response to incident photons. The method also includesforming a charge carrier segregation structure configured to selectivelydirect charge carriers of the plurality of charge carriers into at leastone charge carrier storage region based upon times at which the chargecarriers are produced.

Some embodiments relate to a method of sequencing a nucleic acid. Themethod includes receiving photons from luminescent molecules attached,for at least a period of time, directly or indirectly, to respectivenucleotides of the nucleic acid. The method also includes selectivelydirecting charge carriers of a plurality of charge carriers produced inresponse to the incident photons into at least one charge carrierstorage region based upon times at which the charge carriers areproduced.

Some embodiments relate to a computer readable storage medium havingstored thereon instructions, which when executed by a processor, performa method of sequencing a nucleic acid. The method includes sequencing anucleic acid using, at least in part, arrival times of incident photonsdetected by an integrated circuit that receives the photons fromluminescent molecules connected to respective nucleotides of the nucleicacid.

Some embodiments relate to a method of sequencing a nucleic acid. Themethod includes, using an integrated circuit, detecting arrival times ofincident photons from luminescent molecules connected to respectivenucleotides of the nucleic acid. The method also includes identifyingluminescent molecules using, at least in part, an integrated circuitthat detects arrival times of incident photons from the luminescentmolecules.

Some embodiments relate to a method of fluorescence lifetime imaging.The method includes producing an image indicating fluorescent lifetimesusing, at least in part, an integrated circuit that detects arrivaltimes of incident photons from fluorescent molecules.

Some embodiments relate to a method of time-of-flight imaging. Themethod includes receiving incident photons, and selectively directingcharge carriers of a plurality of charge carriers produced in responseto the incident photons into at least one charge carrier storage regionbased upon times at which the charge carriers are produced.

The foregoing summary is provided by way of illustration and is notintended to be limiting.

BRIEF DESCRIPTION OF DRAWINGS

In the drawings, each identical or nearly identical component that isillustrated in various figures is represented by a like referencecharacter. For purposes of clarity, not every component may be labeledin every drawing. The drawings are not necessarily drawn to scale, withemphasis instead being placed on illustrating various aspects of thetechniques and devices described herein

FIG. 1A plots the probability of a photon being emitted as a function oftime for two markers with different lifetimes.

FIG. 1B shows example intensity profiles over time for an exampleexcitation pulse (dotted line) and example fluorescence emission (solidline).

FIG. 2A shows a diagram of a pixel of an integrated photodetector,according to some embodiments.

FIG. 2B illustrates capturing a charge carrier at a different point intime and space than in FIG. 2A.

FIG. 3A shows a charge carrier confinement region of a pixel, accordingto some embodiments.

FIG. 3B shows the pixel of FIG. 3A with a plurality of electrodesVb0-Vbn, b0-bm, st1, st2, and tx0-tx3 overlying the charge carrierconfinement region of FIG. 3A.

FIG. 3C shows an embodiment in which the photon absorption/carriergeneration region includes a PN junction.

FIG. 3D shows a top view of a pixel as in FIG. 3C, with the addition ofdoping characteristics.

FIG. 3E shows a top view of a pixel as in FIG. 3C, including the carriertravel/capture area.

FIG. 3F shows an array of pixels as in FIG. 3E. FIG. 3F indicatesregions of diffusion, polysilicon, contact and metal 1.

FIG. 3G shows the pixel array of FIG. 3F and also indicates regions ofdiffusion, polysilicon, contact, metal 1, N-implant, P-implant, andP-epi.

FIG. 4 shows a circuit diagram of the pixel of FIG. 3B. The chargecarrier confinement area is shown in heavy dark lines.

FIG. 5A illustrates a potential gradient that may be established in thecharge carrier confinement area in the photon absorption/carriergeneration area and the carrier travel/capture area along the line A-A′of FIG. 3B.

FIG. 5B shows that after a period of time a potential barrier toelectrons may be raised at a time t1 by decreasing the voltage ofelectrode b0.

FIG. 5C shows that after another time period, another potential barrierto electrons may be raised at time t2 by decreasing the voltage ofelectrode b2.

FIG. 5D shows that after another time period, another potential barrierto electrons may be raised at time t3 by decreasing the voltage ofelectrode b4.

FIG. 5E shows that after another time period, another potential barrierto electrons may be raised at time t4 by decreasing the voltage ofelectrode b6.

FIG. 5F shows that after another time period, another potential barrierto electrons may be raised at time t5 by decreasing the voltage ofelectrode bm.

FIG. 6A shows the position of a carrier once it is photogenerated.

FIG. 6B shows the position of a carrier shortly thereafter, as ittravels in the downward direction in response to the establishedpotential gradient.

FIG. 6C shows the position of the carrier as it reaches the drain.

FIG. 6D shows the position of a carrier (e.g., an electron) once it isphotogenerated.

FIG. 6E shows the position of a carrier shortly thereafter, as ittravels in the downward direction in response to the potential gradient.

FIG. 6F shows the position of the carrier as it reaches the potentialbarrier after time t1.

FIG. 6G shows that if an electron arrives between electrodes b0 and b2between times t1 and t2, the electron will be captured between potentialbarrier 501 and potential barrier 502, as illustrated in FIG. 6G.

FIG. 6H shows an example in which an electron arrived between times t1and t2, so it remains captured between potential barrier 501 andpotential barrier 502.

FIG. 6I shows an example in which an electron arrived between times t1and t2, so it remains captured between potential barrier 501 andpotential barrier 502.

FIG. 6J shows an example in which an electron arrived between times t1and t2, so it remains captured between potential barrier 501 andpotential barrier 502.

FIG. 6K shows a voltage timing diagram illustrating the voltages ofelectrodes b0-b8, st0 and st1 over time.

FIG. 7A shows a plot of the of potential for a cross section of thecharge carrier confinement area along the line B-B′ of FIG. 3B.

FIG. 7B shows that after time t5 the voltage on electrodes b1, b3, b5and b7 optionally may be decreased (not shown in FIG. 6K) to raise theposition of an electron within the potential well, to facilitatetransferring the electron.

FIG. 7C shows that at time t6 (FIG. 6K), the voltages on electrodes st0and st1 may be raised.

FIG. 7D shows that at time t7, the voltage on electrode st0 may bedropped, thereby confining the captured carrier (if any) in thecorresponding bin (bin2 in this example).

FIG. 7E shows a plan view illustrating an electron captured betweenpotential barriers 503 and 504.

FIG. 7F shows a plan view illustrating the voltage of electrode st1being raised and the carrier being transferred.

FIG. 7G shows a plan view illustrating the voltage electrode st1 beinglowered and the carrier being captured in bin2.

FIG. 7H shows the characteristics of the electrodes of a charge carriersegregation structure, according to some embodiments.

FIG. 8A shows a flowchart of a method that includes performing aplurality of measurements, according to some embodiments.

FIG. 8B is a diagram showing an excitation pulse being generated at timet0, and time bins bin0-bin3.

FIG. 8C shows a plot of the number of photons/charge carriers in eachtime bin for a set of fluorescence lifetime measurements in which theprobability of a marker or die fluorescing decreases exponentially overtime.

FIG. 8D shows a method of operating the integrated photodetectoraccording to some embodiments in which light is received at theintegrated photodetector in response to a plurality of different triggerevents.

FIG. 8E illustrates voltages of the electrodes of the charge carriersegregation structure when performing the method of FIG. 8D.

FIG. 9A shows an example of a timing diagram for sequentially readingout bins bin0-bin3 using correlated double sampling.

FIG. 9B shows a readout sequence for performing correlated doublesampling that does not require measuring a reset value for each signalvalue, according to some embodiments.

FIG. 10A illustrates an array of pixels having a plurality of columns C1to Cn and a plurality of rows, with a selected row Ri being shown by wayof illustration.

FIG. 10B shows an embodiment in which a common readout circuit may beprovided for a plurality of columns.

FIG. 10C shows an embodiments with a plurality of readout circuits,fewer than the number of columns.

FIG. 10D shows a circuit diagram illustrating column readout circuitrywhich includes sample and hold circuitry, amplifier circuitry and ananalog-to-digital (A/D) converter.

FIG. 10E illustrates an embodiment of readout circuitry in which boththe amplifier circuitry and the A/D converter are shared by two columnsof the pixel array.

FIG. 10F shows an embodiment in which n columns of the pixel array sharereadout circuitry and/or an A/D converter.

FIG. 10G shows an example of amplifier circuitry that includes aplurality of amplifiers.

FIG. 10H shows a diagram of readout circuitry including amplifiercircuitry having first stage amplifiers for respective columns and asecond stage amplifier that is shared by the two columns.

FIG. 10I shows a diagram of readout circuitry including first-stageamplifiers, a second stage amplifier and a third stage amplifier.

FIG. 10J shows readout circuitry shared by two columns including adifferential sample and hold circuit and a differential amplifier.

FIG. 10K shows a diagram of the differential sample and hold circuit anda differential amplifier when the first column is in the sample phaseand the second column is in the hold phase.

FIG. 10L shows a diagram of the differential sample and hold circuit anda differential amplifier when the second column is in the sample phaseand the first column is in the hold phase.

FIG. 10M shows readout circuitry shared by more than two columnsincluding a differential sample and hold circuit and a differentialamplifier.

FIG. 11 shows the timing of the time bins may be controlled adaptivelybetween measurements based on the results of a set of measurements.

FIG. 12 shows an example of a pixel that includes four sub-pixels.

FIG. 13 shows a diagram of a chip architecture, according to someembodiments.

FIG. 14A shows a diagram of an embodiment of a chip having a 64×64 arrayof quad pixels, according to some embodiments.

FIG. 14B shows a diagram of an embodiment of a chip that includes 2×2arrays, with each array having 256×64 octal pixels array of quad pixels,according to some embodiments.

FIG. 15A shows a perspective view of charge confinement regions that maybe formed in a semiconductor substrate.

FIG. 15B shows a plan view corresponding to FIG. 15A.

FIG. 16 shows the formation of electrodes over the insulating layer byforming a patterned polysilicon layer.

FIG. 17 shows a split-doped electrode having a p+ region and an n+region.

FIG. 18 shows the formation of a metal layer (e.g., metal 1) over thepatterned polysilicon layer to connect to the vias.

FIG. 19 shows the metal layer overlaid on the polysilicon layer andcharge confinement regions.

FIG. 20 shows the formation of vias to contact the metal layer.

FIG. 21 shows the second metal layer as well as formation of via(s) tocontact the second metal layer.

FIG. 22 shows the formation of a third metal layer.

FIG. 23 shows an example of a drive circuit for driving an electrode ofthe charge carrier segregation structure, according to some embodiments.

FIG. 24 shows an embodiment in which chip is affixed to a printedcircuit board FIG. 25 illustrates enabling 32 rows in a central regionof the chip and disabling 48 rows at the edges of the chip.

FIG. 26 is a block diagram of an illustrative computing device.

DETAILED DESCRIPTION

Described herein is an integrated photodetector that can accuratelymeasure, or “time-bin,” the timing of arrival of incident photons. Insome embodiments, the integrated photodetector can measure the arrivalof photons with nanosecond or picosecond resolution. Such aphotodetector may find application in a variety of applicationsincluding molecular detection/quantiation, which may be applied tosequencing of nucleic acids (e.g., DNA sequencing). Such a photodetectorcan facilitate time-domain analysis of the arrival of incident photonsfrom luminescent molecules used to label nucleotides, thereby enablingidentification and sequencing of nucleotides based upon luminancelifetimes. Other examples of applications of the integratedphotodetector include fluorescence lifetime imaging and time-of-flightimaging, as discussed further below.

Discussion of Time Domain Measurements for MolecularDetection/Quantitation

Detection and quantitation of biological samples may be performed usingbiological assays (“bioassays”). Bioassays conventionally involve large,expensive laboratory equipment requiring research scientists trained tooperate the equipment and perform the bioassays. Bioassays areconventionally performed in bulk such that a large amount of aparticular type of sample is necessary for detection and quantitation.Some bioassays are performed by tagging samples with luminescent markersthat emit light of a particular wavelength. The samples are illuminatedwith a light source to cause luminescence, and the luminescent light isdetected with a photodetector to quantify the amount of light emitted bythe markers. Bioassays using luminescent tags and/or reportersconventionally involve expensive laser light sources to illuminatesamples and complicated luminescent detection optics and electronics tocollect the light from the illuminated samples.

In some embodiments, an integrated photodetector as described herein candetect the luminance characteristics of biological and/or chemicalsample(s) in response to excitation. More specifically, such anintegrated photodetector can detect the temporal characteristics oflight received from the sample(s). Such an integrated photodetector canenable detecting and/or discriminating the luminance lifetime, e.g., thefluorescence lifetime, of light emitted by a luminescent molecule inresponse to excitation. In some embodiments, identification and/orquantitative measurements of sample(s) can be performed based ondetecting and/or discriminating luminance lifetimes. For example, insome embodiments sequencing of a nucleic acid (e.g., DNA, RNA) may beperformed by detecting and/or discriminating luminance lifetimes ofluminescent molecules attached to respective nucleotides. Eachluminescent molecule may be directly attached (e.g., bonded) to acorresponding nucleotide or indirectly attached to a correspondingnucleotide via a linker molecule that is bonded to the nucleotide andthe luminescent molecule.

In some embodiments, an integrated photodetector having a number ofphotodetection structures and associated electronics, termed “pixels,”can enable measurement and analysis of a plurality of samples inparallel (e.g., hundreds, thousands, millions or more), which can reducethe cost of performing complex measurements and rapidly advance the rateof discoveries. In some embodiments, each pixel of the photodetector maydetect light from a sample, which may be a single molecule or more thanone molecule. In some embodiments, such an integrated photodetector canbe used for dynamic real time applications such as nucleic acid (e.g.,DNA, RNA) sequencing.

Detection/Quantiation of Molecules Using Luminance Lifetimes

An integrated circuit having an integrated photodetector according toaspects of the present application may be designed with suitablefunctions for a variety of detection and imaging applications. Asdescribed in further detail below, such an integrated photodetector canhave the ability to detect light within one or more time intervals, or“time bins.” To collect information regarding the time of arrival of thelight, charge carriers are generated in response to incident photons andcan be segregated into respective time bins based upon their time ofarrival.

An integrated photodetector according to some aspects of the presentapplication may be used for differentiating among light emissionsources, including luminescent molecules, such as fluorophores.Luminescent molecules vary in the wavelength of light they emit, thetemporal characteristics of the light they emit (e.g., their emissiondecay time periods), and their response to excitation energy.Accordingly, luminescent molecules may be identified or discriminatedfrom other luminescent molecules based on detecting these properties.Such identification or discrimination techniques may be used alone or inany suitable combination.

In some embodiments, an integrated photodetector as described in thepresent application can measure or discriminate luminance lifetimes,such as fluorescence lifetimes. Fluorescence lifetime measurements arebased on exciting one or more fluorescent molecules, and measuring thetime variation in the emitted luminescence. The probability of afluorescent molecule to emit a photon after the fluorescent moleculereaches an excited state decreases exponentially over time. The rate atwhich the probability decreases may be characteristic of a fluorescentmolecule, and may be different for different fluorescent molecules.Detecting the temporal characteristics of light emitted by fluorescentmolecules may allow identifying fluorescent molecules and/ordiscriminating fluorescent molecules with respect to one another.Luminescent molecules are also referred to herein as luminescentmarkers, or simply “markers.”

After reaching an excited state, a marker may emit a photon with acertain probability at a given time. The probability of a photon beingemitted from an excited marker may decrease over time after excitationof the marker. The decrease in the probability of a photon being emittedover time may be represented by an exponential decay functionp(t)=e^(−t)/τ, where p(t) is the probability of photon emission at atime, t, and τ is a temporal parameter of the marker. The temporalparameter τ indicates a time after excitation when the probability ofthe marker emitting a photon is a certain value. The temporal parameter,τ, is a property of a marker that may be distinct from its absorptionand emission spectral properties. Such a temporal parameter, τ, isreferred to as the luminance lifetime, the fluorescence lifetime orsimply the “lifetime” of a marker.

FIG. 1A plots the probability of a photon being emitted as a function oftime for two markers with different lifetimes. The marker represented byprobability curve B has a probability of emission that decays morequickly than the probability of emission for the marker represented byprobability curve A. The marker represented by probability curve B has ashorter temporal parameter, τ, or lifetime than the marker representedby probability curve A. Markers may have fluorescence lifetimes rangingfrom 0.1-20 ns, in some embodiments. However, the techniques describedherein are not limited as to the lifetimes of the marker(s) used.

The lifetime of a marker may be used to distinguish among more than onemarker, and/or may be used to identify marker(s). In some embodiments,fluorescence lifetime measurements may be performed in which a pluralityof markers having different lifetimes are excited by an excitationsource. As an example, four markers having lifetimes of 0.5, 1, 2, and 3nanoseconds, respectively, may be excited by a light source that emitslight having a selected wavelength (e.g., 635 nm, by way of example).The markers may be identified or differentiated from each other based onmeasuring the lifetime of the light emitted by the markers.

Fluorescence lifetime measurements may use relative intensitymeasurements by comparing how intensity changes over time, as opposed toabsolute intensity values. As a result, fluorescence lifetimemeasurements may avoid some of the difficulties of absolute intensitymeasurements. Absolute intensity measurements may depend on theconcentration of fluorophores present and calibration steps may beneeded for varying fluorophore concentrations. By contrast, fluorescencelifetime measurements may be insensitive to the concentration offluorophores.

Luminescent markers may be exogenous or endogenous. Exogenous markersmay be external luminescent markers used as a reporter and/or tag forluminescent labeling. Examples of exogenous markers may includefluorescent molecules, fluorophores, fluorescent dyes, fluorescentstains, organic dyes, fluorescent proteins, enzymes, and/or quantumdots. Such exogenous markers may be conjugated to a probe or functionalgroup (e.g., molecule, ion, and/or ligand) that specifically binds to aparticular target or component. Attaching an exogenous tag or reporterto a probe allows identification of the target through detection of thepresence of the exogenous tag or reporter. Examples of probes mayinclude proteins, nucleic acids such as DNA molecules or RNA molecules,lipids and antibody probes. The combination of an exogenous marker and afunctional group may form any suitable probes, tags, and/or labels usedfor detection, including molecular probes, labeled probes, hybridizationprobes, antibody probes, protein probes (e.g., biotin-binding probes),enzyme labels, fluorescent probes, fluorescent tags, and/or enzymereporters.

While exogenous markers may be added to a sample or region, endogenousmarkers may be already part of the sample or region. Endogenous markersmay include any luminescent marker present that may luminesce or“autofluoresce” in the presence of excitation energy. Autofluorescenceof endogenous fluorophores may provide for label-free and noninvasivelabeling without requiring the introduction of endogenous fluorophores.Examples of such endogenous fluorophores may include hemoglobin,oxyhemoglobin, lipids, collagen and elastin crosslinks, reducednicotinamide adenine dinucleotide (NADH), oxidized flavins (FAD andFMN), lipofuscin, keratin, and/or prophyrins, by way of example and notlimitation.

Differentiating between markers by lifetime measurements may allow forfewer wavelengths of excitation light to be used than when the markersare differentiated by measurements of emission spectra. In someembodiments, sensors, filters, and/or diffractive optics may be reducedin number or eliminated when using fewer wavelengths of excitation lightand/or luminescent light. In some embodiments, labeling may be performedwith markers that have different lifetimes, and the markers may beexcited by light having the same excitation wavelength or spectrum. Insome embodiments, an excitation light source may be used that emitslight of a single wavelength or spectrum, which may reduce the cost.However, the techniques described herein are not limited in thisrespect, as any number of excitation light wavelengths or spectra may beused. In some embodiments, an integrated photodetector may be used todetermine both spectral and temporal information regarding receivedlight. In some embodiments a quantitative analysis of the types ofmolecule(s) present may be performed by determining a temporalparameter, a spectral parameter, or a combination of the temporal andspectral parameters of the emitted luminescence from a marker.

An integrated photodetector that detects the arrival time of incidentphotons may reduce additional optical filtering (e.g., optical spectralfiltering) requirements. As described below, an integrated photodetectoraccording to the present application may include a drain to removephotogenerated carriers at particular times. By removing photogeneratedcarriers in this manner, unwanted charge carriers produced in responseto an excitation light pulse may be discarded without the need foroptical filtering to prevent reception of light from the excitationpulse. Such a photodetector may reduce overall design integrationcomplexity, optical and/or filtering components, and/or cost.

In some embodiments, a fluorescence lifetime may be determined bymeasuring the time profile of the emitted luminescence by aggregatingcollected charge carriers in one or more time bins of the integratedphotodetector to detect luminance intensity values as a function oftime. In some embodiments, the lifetime of a marker may be determined byperforming multiple measurements where the marker is excited into anexcited state and then the time when a photon emits is measured. Foreach measurement, the excitation source may generate a pulse ofexcitation light directed to the marker, and the time between theexcitation pulse and subsequent photon event from the marker may bedetermined. Additionally or alternatively, when an excitation pulseoccurs repeatedly and periodically, the time between when a photonemission event occurs and the subsequent excitation pulse may bemeasured, and the measured time may be subtracted from the time intervalbetween excitation pulses (i.e., the period of the excitation pulsewaveform) to determine the time of the photon absorption event.

By repeating such experiments with a plurality of excitation pulses, thenumber of instances a photon is emitted from the marker within a certaintime interval after excitation may be determined, which is indicative ofthe probability of a photon being emitted within such a time intervalafter excitation. The number of photon emission events collected may bebased on the number of excitation pulses emitted to the marker. Thenumber of photon emission events over a measurement period may rangefrom 50-10,000,000 or more, in some embodiments, however, the techniquesdescribed herein are not limited in this respect. The number ofinstances a photon is emitted from the marker within a certain timeinterval after excitation may populate a histogram representing thenumber of photon emission events that occur within a series of discretetime intervals or time bins. The number of time bins and/or the timeinterval of each bin may be set and/or adjusted to identify a particularlifetime and/or a particular marker. The number of time bins and/or thetime interval of each bin may depend on the sensor used to detect thephotons emitted. The number of time bins may be 1, 2, 3, 4, 5, 6, 7, 8,or more, such as 16, 32, 64, or more. A curve fitting algorithm may beused to fit a curve to the recorded histogram, resulting in a functionrepresenting the probability of a photon to be emitted after excitationof the marker at a given time. An exponential decay function, such asp(t)=e^(−t/τ), may be used to approximately fit the histogram data. Fromsuch a curve fitting, the temporal parameter or lifetime may bedetermined. The determined lifetime may be compared to known lifetimesof markers to identify the type of marker present.

A lifetime may be calculated from the intensity values at two timeintervals. FIG. 1B shows example intensity profiles over time for anexample excitation pulse (dotted line) and example fluorescence emission(solid line). In the example shown in FIG. 1B, the photodetectormeasures the intensity over at least two time bins. The photons thatemit luminescence energy between times t1 and t2 are measured by thephotodetector as intensity I1 and luminescence energy emitted betweentimes t3 and t4 are measured as I2. Any suitable number of intensityvalues may be obtained although only two are shown in FIG. 1B. Suchintensity measurements may then be used to calculate a lifetime. Whenone fluorophore is present at a time, then the time binned luminescencesignal may be fit to a single exponential decay. In some embodiments,only two time bins may be needed to accurately identify the lifetime fora fluorophore. When two or more fluorophores are present, thenindividual lifetimes may be identified from a combined luminescencesignal by fitting the luminescence signal to multiple exponentialdecays, such as double or triple exponentials. In some embodiments twoor more time bins may be needed in order to accurately identify morethan one fluorescence lifetime from such a luminescence signal. However,in some instances with multiple fluorophores, an average fluorescencelifetime may be determined by fitting a single exponential decay to theluminescence signal.

In some instances, the probability of a photon emission event and thusthe lifetime of a marker may change based on the surroundings and/orconditions of the marker. For example, the lifetime of a marker confinedin a volume with a diameter less than the wavelength of the excitationlight may be smaller than when the marker is not in the volume. Lifetimemeasurements with known markers under conditions similar to when themarkers are used for labeling may be performed. The lifetimes determinedfrom such measurements with known markers may be used when identifying amarker.

Sequencing Using Luminance Lifetime Measurements

Individual pixels on an integrated photodetector may be capable offluorescence lifetime measurements used to identify fluorescent tagsand/or reporters that label one or more targets, such as molecules orspecific locations on molecules. Any one or more molecules of interestmay be labeled with a fluorophore, including proteins, amino acids,enzymes, lipids, nucleotides, DNA, and RNA. When combined with detectingspectra of the emitted light or other labeling techniques, fluorescencelifetime may increase the total number of fluorescent tags and/orreporters that can be used. Identification based on lifetime may be usedfor single molecule analytical methods to provide information aboutcharacteristics of molecular interactions in complex mixtures where suchinformation would be lost in ensemble averaging and may includeprotein-protein interactions, enzymatic activity, molecular dynamics,and/or diffusion on membranes. Additionally, fluorophores with differentfluorescence lifetimes may be used to tag target components in variousassay methods that are based on presence of a labeled component. In someembodiments, components may be separated, such as by using microfluidicsystems, based on detecting particular lifetimes of fluorophores.

Measuring fluorescence lifetimes may be used in combination with otheranalytical methods. For an example, fluorescence lifetimes may be usedin combination with fluorescence resonance energy transfer (FRET)techniques to discriminate between the states and/or environments ofdonor and acceptor fluorophores located on one or more molecules. Suchmeasurements may be used to determine the distance between the donor andthe acceptor. In some instances, energy transfer from the donor to theacceptor may decrease the lifetime of the donor. In another example,fluorescence lifetime measurements may be used in combination with DNAsequencing techniques where four fluorophores having different lifetimesmay be used to label the four different nucleotides (A, T, G, C) in aDNA molecule with an unknown sequence of nucleotides. The fluorescencelifetimes, instead of emission spectra, of the fluorophores may be usedto identify the sequence of nucleotides. By using fluorescence lifetimeinstead of emission spectra for certain techniques, accuracy andmeasurement resolution may increase because artifacts due to absoluteintensity measurements are reduced. Additionally, lifetime measurementsmay reduce the complexity and/or expense of the system because fewerexcitation energy wavelengths are required and/or fewer emission energywavelengths need be detected.

The methods described herein may be used for sequencing of nucleicacids, such as DNA sequencing or RNA sequencing. DNA sequencing allowsfor the determination of the order and position of nucleotides in atarget nucleic acid molecule. Technologies used for DNA sequencing varygreatly in the methods used to determine the nucleic acid sequence aswell as in the rate, read length, and incidence of errors in thesequencing process. A number of DNA sequencing methods are based onsequencing by synthesis, in which the identity of a nucleotide isdetermined as the nucleotide is incorporated into a newly synthesizedstrand of nucleic acid that is complementary to the target nucleic acid.Many sequencing by synthesis methods require the presence of apopulation of target nucleic acid molecules (e.g., copies of a targetnucleic acid) or a step of amplification of the target nucleic acid toachieve a population of target nucleic acids. Improved methods fordetermining the sequence of single nucleic acid molecules is desired.

There have been recent advances in sequencing single nucleic acidmolecules with high accuracy and long read length. The target nucleicacid used in single molecule sequencing technology, for example the SMRTtechnology developed by Pacific Biosciences, is a single stranded DNAtemplate that is added to a sample well containing at least onecomponent of the sequencing reaction (e.g., the DNA polymerase)immobilized or attached to a solid support such as the bottom of thesample well. The sample well also contains deoxyribonucleosidetriphosphates, also referred to a “dNTPs,” including adenine, cytosine,guanine, and thymine dNTPs, that are conjugated to detection labels,such as fluorophores. Preferably each class of dNTPs (e.g. adeninedNTPs, cytosine dNTPs, guanine dNTPs, and thymine dNTPs) are eachconjugated to a distinct detection label such that detection of thesignal indicates the identity of the dNTP that was incorporated into thenewly synthesized nucleic acid. The detection label may be conjugated tothe dNTP at any position such that the presence of the detection labeldoes not inhibit the incorporation of the dNTP into the newlysynthesized nucleic acid strand or the activity of the polymerase. Insome embodiments, the detection label is conjugated to the terminalphosphate (the gamma phosphate) of the dNTP.

Any polymerase may be used for single molecule DNA sequencing that iscapable of synthesizing a nucleic acid complementary to a target nucleicacid. Examples of polymerases include E. coli DNA polymerase I, T7 DNApolymerase, bacteriophage T4 DNA polymerase φ29 (psi29) DNA polymerase,and variants thereof. In some embodiments, the polymerase is a singlesubunit polymerase. Upon base pairing between a nucleobase of a targetnucleic acid and the complementary dNTP, the polymerase incorporates thedNTP into the newly synthesized nucleic acid strand by forming aphosphodiester bond between the 3′ hydroxyl end of the newly synthesizedstrand and the alpha phosphate of the dNTP. In examples in which thedetection label conjugated to the dNTP is a fluorophore, its presence issignaled by excitation and a pulse of emission is detected during thestep of incorporation. For detection labels that are conjugated to theterminal (gamma) phosphate of the dNTP, incorporation of the dNTP intothe newly synthesized strand results in release the beta and gammaphosphates and the detection label, which is free to diffuse in thesample well, resulting in a decrease in emission detected from thefluorophore.

The techniques described herein are not limited as to the detection orquantitation of molecules or other samples, or to performing sequencing.In some embodiments, an integrated photodetector may perform imaging toobtain spatial information regarding a region, object or scene andtemporal information regarding the arrival of incident photons using theregion, object or scene. In some embodiments, the integratedphotodetector may perform luminescence lifetime imaging of a region,object or sample, such as fluorescence lifetime imaging.

Additional Applications

Although the integrated photodetector described herein may be applied tothe analysis of a plurality of biological and/or chemical samples, asdiscussed above, the integrated photodetector may be applied to otherapplications, such as imaging applications, for example. In someembodiments, the integrated photodetector may include a pixel array thatperforms imaging of a region, object or scene, and may detect temporalcharacteristics of the light received at individual pixels fromdifferent regions of the region, object or scene. For example, in someembodiments the integrated photodetector may perform imaging of tissuebased on the temporal characteristics of light received from the tissue,which may enable a physician performing a procedure (e.g., surgery) toidentify an abnormal or diseased region of tissue (e.g., cancerous orpre-cancerous). In some embodiments, the integrated photodetector may beincorporated into a medical device, such as a surgical imaging tool. Insome embodiments, time-domain information regarding the light emitted bytissue in response to a light excitation pulse may be obtained to imageand/or characterize the tissue. For example, imaging and/orcharacterization of tissue or other objects may be performed usingfluorescence lifetime imaging.

Although the integrated photodetector may be applied in a scientific ordiagnostic context such as by performing imaging or analysis ofbiological and/or chemical samples, or imaging tissue, as describedabove, such an integrated photodetector may be used in any othersuitable contexts. For example, in some embodiments, such an integratedphotodetector may image a scene using temporal characteristics of thelight detected in individual pixels. An example of an application forimaging a scene is range imaging or time-of-flight imaging, in which theamount of time light takes to reach the photodetector is analyzed todetermine the distance traveled by the light to the photodetector. Sucha technique may be used to perform three-dimensional imaging of a scene.For example, a scene may be illuminated with a light pulse emitted froma known location relative to the integrated photodetector, and thereflected light detected by the photodetector. The amount of time thatthe light takes to reach the integrated photodetector at respectivepixels of the array is measured to determine the distance(s) lighttraveled from respective portions of the scene to reach respectivepixels of the photodetector. In some embodiments, the integratedphotodetector may be incorporated into a consumer electronic device suchas a camera, cellular telephone, or tablet computer, for example, toenable such devices to capture and process images or video based on therange information obtained.

In some embodiments, the integrated photodetector described in thepresent application may be used to measure low light intensities. Such aphotodetector may be suitable for applications that requirephotodetectors with a high sensitivity, such as applications that maycurrently use single photon counting techniques, for example. However,the techniques described herein are not limited in this respect, as theintegrated photodetector described in the present applications maymeasure any suitable light intensities.

Additional Luminescence Lifetime Applications

Imaging and Characterization Using Lifetimes

As mentioned above, the techniques described herein are not limited tolabeling, detection and quantitation using exogenous fluorophores. Insome embodiments, a region, object or sample may be imaged and/orcharacterized using fluorescence lifetime imaging techniques though useof an integrated photodetector. In such techniques, the fluorescencecharacteristics of the region, object or sample itself may be used forimaging and/or characterization. Either exogenous markers or endogenousmarkers may be detected through lifetime imaging and/orcharacterization. Exogenous markers attached to a probe may be providedto the region, object, or sample in order to detect the presence and/orlocation of a particular target component. The exogenous marker mayserve as a tag and/or reporter as part of a labeled probe to detectportions of the region, object, or sample that contains a target for thelabeled probe. Autofluorescence of endogenous markers may provide alabel-free and noninvasive contrast for spatial resolution that can bereadily utilized for imaging without requiring the introduction ofendogenous markers. For example, autofluorescence signals frombiological tissue may depend on and be indicative of the biochemical andstructural composition of the tissue.

Fluorescence lifetime measurements may provide a quantitative measure ofthe conditions surrounding the fluorophore. The quantitative measure ofthe conditions may be in addition to detection or contrast. Thefluorescence lifetime for a fluorophore may depend on the surroundingenvironment for the fluorophore, such as pH or temperature, and a changein the value of the fluorescence lifetime may indicate a change in theenvironment surrounding the fluorophore. As an example, fluorescencelifetime imaging may map changes in local environments of a sample, suchas in biological tissue (e.g., a tissue section or surgical resection).Fluorescence lifetime measurements of autofluorescence of endogenousfluorophores may be used to detect physical and metabolic changes in thetissue. As examples, changes in tissue architecture, morphology,oxygenation, pH, vascularity, cell structure and/or cell metabolic statemay be detected by measuring autofluorescence from the sample anddetermining a lifetime from the measured autofluorescence. Such methodsmay be used in clinical applications, such as screening, image-guidedbiopsies or surgeries, and/or endoscopy. In some embodiments, anintegrated photodetector of the present application may be incorporatedinto a clinical tool, such as a surgical instrument, for example, toperform fluorescence lifetime imaging. Determining fluorescencelifetimes based on measured autofluorescence provides clinical value asa label-free imaging method that allows a clinician to quickly screentissue and detect small cancers and/or pre-cancerous lesions that arenot apparent to the naked eye. Fluorescence lifetime imaging may be usedfor detection and delineation of malignant cells or tissue, such astumors or cancer cells which emit luminescence having a longerfluorescence lifetime than healthy tissue. For example, fluorescencelifetime imaging may be used for detecting cancers on opticallyaccessible tissue, such as gastrointestinal tract, bladder, skin, ortissue surface exposed during surgery.

In some embodiments, fluorescence lifetimes may be used for microscopytechniques to provide contrast between different types or states ofsamples. Fluorescence lifetime imaging microscopy (FLIM) may beperformed by exciting a sample with a light pulse, detecting thefluorescence signal as it decays to determine a lifetime, and mappingthe decay time in the resulting image. In such microscopy images, thepixel values in the image may be based on the fluorescence lifetimedetermined for each pixel in the photodetector collecting the field ofview.

Imaging a Scene or Object Using Temporal Information

As discussed above, an integrated photodetector as described in thepresent application may be used in scientific and clinical contexts inwhich the timing of light emitted may be used to detect, quantify, andor image a region, object or sample. However, the techniques describedherein are not limited to scientific and clinical applications, as theintegrated photodetector may be used in any imaging application that maytake advantage of temporal information regarding the time of arrival ofincident photons. An example of an application is time-of-flightimaging.

Time-of-Flight Applications

In some embodiments, an integrated photodetector may be used in imagingtechniques that are based on measuring a time profile of scattered orreflected light, including time-of-flight measurements. In suchtime-of-flight measurements, a light pulse may be is emitted into aregion or sample and scattered light may be detected by the integratedphotodetector. The scattered or reflected light may have a distinct timeprofile that may indicate characteristics of the region or sample.Backscattered light by the sample may be detected and resolved by theirtime of flight in the sample. Such a time profile may be a temporalpoint spread function (TPSF). The time profile may be acquired bymeasuring the integrated intensity over multiple time bins after thelight pulse is emitted. Repetitions of light pulses and accumulating thescattered light may be performed at a certain rate to ensure that allthe previous TPSF is completely extinguished before generating asubsequent light pulse. Time-resolved diffuse optical imaging methodsmay include spectroscopic diffuse optical tomography where the lightpulse may be infrared light in order to image at a further depth in thesample. Such time-resolved diffuse optical imaging methods may be usedto detect tumors in an organism or in part of an organism, such as aperson's head.

Additionally or alternatively, time-of-flight measurements may be usedto measure distance or a distance range based on the speed of light andtime between an emitted light pulse and detecting light reflected froman object. Such time-of-flight techniques may be used in a variety ofapplications including cameras, proximity detection sensors inautomobiles, human-machine interfaces, robotics and other applicationsthat may use three-dimensional information collected by such techniques.

Integrated Photodetector for Time Binning Photogenerated Charge Carriers

Some embodiments relate to an integrated circuit having a photodetectorthat produces charge carriers in response to incident photons and whichis capable of discriminating the timing at which the charge carriers aregenerated by the arrival of incident photons with respect to a referencetime (e.g., a trigger event). In some embodiments, a charge carriersegregation structure segregates charge carriers generated at differenttimes and directs the charge carriers into one or more charge carrierstorage regions (termed “bins”) that aggregate charge carriers producedwithin different time periods. Each bin stores charge carriers producedwithin a selected time interval. Reading out the charge stored in eachbin can provide information about the number of photons that arrivedwithin each time interval. Such an integrated circuit can be used in anyof a variety of applications, such as those described herein.

An example of an integrated circuit having a photodetection region and acharge carrier segregation structure will be described. In someembodiments, the integrated circuit may include an array of pixels, andeach pixel may include one or more photodetection regions and one ormore charge carrier segregation structures, as discussed below.

Overview of Pixel Structure and Operation

FIG. 2A shows a diagram of a pixel 100, according to some embodiments.Pixel 100 includes a photon absorption/carrier generation region 102(also referred to as a photodetection region), a carrier travel/captureregion 106, a carrier storage region 108 having one or more chargecarrier storage regions, also referred to herein as “charge carrierstorage bins” or simply “bins,” and readout circuitry 110 for readingout signals from the charge carrier storage bins.

The photon absorption/carrier generation region 102 may be a region ofsemiconductor material (e.g., silicon) that can convert incident photonsinto photogenerated charge carriers. The photon absorption/carriergeneration region 102 may be exposed to light, and may receive incidentphotons. When a photon is absorbed by the photon absorption/carriergeneration region 102 it may generate photogenerated charge carriers,such as an electron/hole pair. Photogenerated charge carriers are alsoreferred to herein simply as “charge carriers.”

An electric field may be established in the photon absorption/carriergeneration region 102. In some embodiments, the electric field may be“static,” as distinguished from the changing electric field in thecarrier travel/capture region 106. The electric field in the photonabsorption/carrier generation region 102 may include a lateralcomponent, a vertical component, or both a lateral and a verticalcomponent. The lateral component of the electric field may be in thedownward direction of FIG. 2A, as indicated by the arrows, which inducesa force on photogenerated charge carriers that drives them toward thecarrier travel/capture region 106. The electric field may be formed in avariety of ways.

In some embodiments one or more electrodes may be formed over the photonabsorption/carrier generation region 102. The electrodes(s) may havevoltages applied thereto to establish an electric field in the photonabsorption/carrier generation region 102. Such electrode(s) may betermed “photogate(s).” In some embodiments, photon absorption/carriergeneration region 102 may be a region of silicon that is fully depletedof charge carriers.

In some embodiments, the electric field in the photon absorption/carriergeneration region 102 may be established by a junction, such as a PNjunction. The semiconductor material of the photon absorption/carriergeneration region 102 may be doped to form the PN junction with anorientation and/or shape that produces an electric field that induces aforce on photogenerated charge carriers that drives them toward thecarrier travel/capture region 106. Producing the electric field using ajunction may improve the quantum efficiency with respect to use ofelectrodes overlying the photon absorption/carrier generation region 102which may prevent a portion of incident photons from reaching the photonabsorption/carrier generation region 102. Using a junction may reducedark current with respect to use of photogates. It has been appreciatedthat dark current may be generated by imperfections at the surface ofthe semiconductor substrate that may produce carriers. In someembodiments, the P terminal of the PN junction diode may connected to aterminal that sets its voltage. Such a diode may be referred to as a“pinned” photodiode. A pinned photodiode may promote carrierrecombination at the surface, due to the terminal that sets its voltageand attracts carriers, which can reduce dark current. Photogeneratedcharge carriers that are desired to be captured may pass underneath therecombination area at the surface. In some embodiments, the lateralelectric field may be established using a graded doping concentration inthe semiconductor material.

In some embodiments, a absorption/carrier generation region 102 that hasa junction to produce an electric field may have one or more of thefollowing characteristics:

-   -   1) a depleted n-type region that is tapered away from the time        varying field,    -   2) a p-type implant surrounding the n-type region with a gap to        transition the electric field laterally into the n-type region,        and/or    -   3) a p-type surface implant that buries the n-type region and        serves as a recombination region for parasitic electrons.

In some embodiments, the electric field may be established in the photonabsorption/carrier generation region 102 by a combination of a junctionand at least one electrode. For example, a junction and a singleelectrode, or two or more electrodes, may be used. In some embodiments,one or more electrodes may be positioned near carrier travel/captureregion 106 to establish the potential gradient near carriertravel/capture region 106, which may be positioned relatively far fromthe junction.

As illustrated in FIG. 2A, a photon may be captured and a charge carrier101A (e.g., an electron) may be produced at time t1. In someembodiments, an electrical potential gradient may be established alongthe photon absorption/carrier generation region 102 and the carriertravel/capture region 106 that causes the charge carrier 101A to travelin the downward direction of FIG. 2A (as illustrated by the arrows shownin FIG. 2A). In response to the potential gradient, the charge carrier101A may move from its position at time t1 to a second position at timet2, a third position at time t3, a fourth position at time t4, and afifth position at time t5. The charge carrier 101A thus moves into thecarrier travel/capture region 106 in response to the potential gradient.

The carrier travel/capture region 106 may be a semiconductor region. Insome embodiments, the carrier travel/capture region 106 may be asemiconductor region of the same material as photon absorption/carriergeneration region 102 (e.g., silicon) with the exception that carriertravel/capture region 106 may be shielded from incident light (e.g., byan overlying opaque material, such as a metal layer).

In some embodiments, and as discussed further below, a potentialgradient may be established in the photon absorption/carrier generationregion 102 and the carrier travel/capture region 106 by electrodespositioned above these regions. An example of the positioning ofelectrodes will be discussed with reference to FIG. 3B. However, thetechniques described herein are not limited as to particular positionsof electrodes used for producing an electric potential gradient. Nor arethe techniques described herein limited to establishing an electricpotential gradient using electrodes. In some embodiments, an electricpotential gradient may be established using a spatially graded dopingprofile and/or a PN junction. Any suitable technique may be used forestablishing an electric potential gradient that causes charge carriersto travel along the photon absorption/carrier generation region 102 andcarrier travel/capture region 106.

A charge carrier segregation structure may be formed in the pixel toenable segregating charge carriers produced at different times. In someembodiments, at least a portion of the charge carrier segregationstructure may be formed over the carrier travel/capture region 106. Aswill be described below, the charge carrier segregation structure mayinclude one or more electrodes formed over the carrier travel/captureregion 106, the voltage of which may be controlled by control circuitryto change the electric potential in the carrier travel/capture region106.

The electric potential in the carrier travel/capture region 106 may bechanged to enable capturing a charge carrier. The potential gradient maybe changed by changing the voltage on one or more electrodes overlyingthe carrier travel/capture region 106 to produce a potential barrierthat can confine a carrier within a predetermined spatial region. Forexample, the voltage on an electrode overlying the dashed line in thecarrier travel/capture region 106 of FIG. 2A may be changed at time t5to raise a potential barrier along the dashed line in the carriertravel/capture region 106 of FIG. 2A, thereby capturing charge carrier101A. As shown in FIG. 2A, the carrier captured at time t5 may betransferred to a bin “bin0” of carrier storage region 108. The transferof the carrier to the charge carrier storage bin may be performed bychanging the potential in the carrier travel/capture region 106 and/orcarrier storage region 108 (e.g., by changing the voltage ofelectrode(s) overlying these regions) to cause the carrier to travelinto the charge carrier storage bin.

Changing the potential at a certain point in time within a predeterminedspatial region of the carrier travel/capture region 106 may enabletrapping a carrier that was generated by photon absorption that occurredwithin a specific time interval. By trapping photogenerated chargecarriers at different times and/or locations, the times at which thecharge carriers were generated by photon absorption may bediscriminated. In this sense, a charge carrier may be “time binned” bytrapping the charge carrier at a certain point in time and/or spaceafter the occurrence of a trigger event. The time binning of a chargecarrier within a particular bin provides information about the time atwhich the photogenerated charge carrier was generated by absorption ofan incident photon, and thus likewise “time bins,” with respect to thetrigger event, the arrival of the incident photon that produced thephotogenerated charge carrier.

FIG. 2B illustrates capturing a charge carrier at a different point intime and space. As shown in FIG. 2B, the voltage on an electrodeoverlying the dashed line in the carrier travel/capture region 106 maybe changed at time t9 to raise a potential barrier along the dashed linein the carrier travel/capture region 106 of FIG. 2B, thereby capturingcarrier 101B. As shown in FIG. 2B, the carrier captured at time t9 maybe transferred to a bin “bin1” of carrier storage region 108. Sincecharge carrier 101B is trapped at time t9, it represents a photonabsorption event that occurred at a different time (i.e., time t6) thanthe photon absorption event (i.e., at t1) for carrier 101A, which iscaptured at time t5.

Performing multiple measurements and aggregating charge carriers in thecharge carrier storage bins of carrier storage region 108 based on thetimes at which the charge carriers are captured can provide informationabout the times at which photons are captured in the photonabsorption/carrier generation area 102. Such information can be usefulin a variety of applications, as discussed above.

Detailed Example of Pixel Structure and Operation

FIG. 3A shows a charge carrier confinement region 103 of a pixel 100A,according to some embodiments. As illustrated in FIG. 3A, pixel 100A mayinclude a photon absorption/carrier generation area 102A (also referredto as a photodetection region), a carrier travel/capture area 106A, adrain 104, a plurality of charge carrier storage bins bin0, bin1, bin2,and bin3 of a carrier storage region 108A, and a readout region 110A.

Charge carrier confinement region 103 is a region in whichphotogenerated charge carriers move in response to the electricpotential gradient produced by a charge carrier segregation structure.Charge carriers may be generated in photon absorption/carrier generationarea 102A within charge carrier confinement region 103.

Charge carrier confinement region 103 may be formed of any suitablematerial, such as a semiconductor material (e.g., silicon). However, thetechniques described herein are not limited in this respect, as anysuitable material may form charge carrier confinement region 103. Insome embodiments, charge carrier confinement region 103 may besurrounded by an insulator (e.g., silicon oxide) to confine chargecarriers within charge carrier confinement region 103.

The portion of charge carrier confinement region 103 in photonabsorption/carrier generation area 102A may have any suitable shape. Asshown in FIG. 3A, in some embodiments the portion of charge carrierconfinement region 103 in photon absorption/carrier generation area 102Amay have a tapered shape, such that its width gradually decreases nearcarrier travel/capture area 106A. Such a shape may improve theefficiency of charge handling, which may be useful particularly in caseswhere few photons are expected to arrive. In some embodiments theportion of charge carrier confinement region 103 in photonabsorption/carrier generation area 102A may be less tapered, or may notbe tapered, which can increase the dynamic range. However, thetechniques described herein are not limited as to the shape of chargecarrier confinement region 103 in photon absorption/carrier generationarea 102A.

As shown in FIG. 3A, a first portion of charge carrier confinementregion 103 in carrier travel/capture area 106A may extend from thephoton absorption/carrier generation area 102A to a drain 104.Extensions of the charge carrier confinement region 103 extend to therespective charge storage bins, allowing charge carriers to be directedinto the charge carrier storage bins by a charge carrier segregationstructure such as that described with respect to FIG. 3B. In someembodiments, the number of extensions of the charge carrier confinementregion 103 that are present may be the same as the number of chargecarrier storage bins, with each extension extending to a respectivecharge carrier storage bin.

Readout region 110A may include a floating diffusion node fd for readout of the charge storage bins. Floating diffusion node fd may be formedby a diffusion of n-type dopants into a p-type material (e.g., a p-typesubstrate), for example. However, the techniques described herein arenot limited as to particular dopant types or doping techniques.

FIG. 3B shows the pixel 100A of FIG. 3A with a plurality of electrodesVb0-Vbn, b0-bm, st1, st2, and tx0-tx3 overlying the charge carrierconfinement region 103 of FIG. 3A. The electrodes shown in FIG. 3B format least a portion of a charge carrier segregation structure that cantime-bin photogenerated carriers.

The electrodes shown in FIG. 3B establish an electric potential withinthe charge carrier confinement region 103. In some embodiments, theelectrodes Vb0-Vbn, b0-bm may have a voltage applied thereto toestablish a potential gradient within regions 102A and 106A such thatcharge carriers, e.g., electrons, travel in the downward direction ofFIG. 3B toward the drain 104. Electrodes Vb0-Vbn may establish apotential gradient in the charge confinement region 103 of photonabsorption/carrier generation area 102A. In some embodiments, respectiveelectrodes Vb0-Vbn may be at constant voltages. Electrodes b0-bm mayestablish a potential gradient in the charge confinement region 103 ofcarrier travel/capture area 106A. In some embodiments, electrodes b0-bmmay have their voltages set to different levels to enable trappingcharge carriers and/or transferring charge carriers to one or morecharge storage bins.

Electrodes st0 and st1 may have voltages that change to transfercarriers to the charge storage bins of charge carrier storage region108A. Transfer gates tx0, tx1, tx2 and tx3 enable transfer of chargefrom the charge storage bins to the floating diffusion node fd. Readoutcircuitry 110 including reset transistor rt, amplification transistor sfand selection transistor rs is also shown.

In some embodiments, the potentials of floating diffusion node fd andeach of the transfer gates tx0-tx3 may allow for overflow of chargecarriers into the floating diffusion rather than into the carriertravel/capture area 106A. When charge carriers are transferred into abin within the carrier storage region 108, the potentials of thefloating diffusion node fd and the transfer gates tx0-tx3 may besufficiently high to allow any overflow charge carriers in the bin toflow to the floating diffusion. Such a “barrier overflow protection”technique may reduce carriers overflowing and diffusing into the carriertravel/capture area 106A and/or other areas of the pixel. In someembodiments, a barrier overflow protection technique may be used toremove any overflow charge carriers generated by an excitation pulse. Byallowing overflow charge carriers to flow to the floating diffusion,these charge carriers are not captured in one or more time bins, therebyreducing the impact of the excitation pulse on the time bin signalsduring readout.

In some embodiments in which electrodes Vb0-Vbn and b0-bm are disposedover the photon absorption/carrier generation region 102 and/or thecarrier travel/capture region 106, the electrodes Vb0-Vbn and b0-bm maybe set to voltages that increase for positions progressing from the topto the bottom of FIG. 3B, thereby establishing the potential gradientthat causes charge carriers to travel in the downward direction of FIG.3B toward the drain 104. In some embodiments, the potential gradient mayvary monotonically in the photon absorption/carrier generation region102 and/or the carrier travel/capture region 106, which may enablecharge carriers to travel along the potential gradient into the carriertravel/capture region 106. In some embodiments, the potential gradientmay change linearly with respect to position along the line A-A′. Alinear potential gradient may be established by setting electrodes tovoltages that vary linearly across the vertical dimension of FIG. 3B.However, the techniques described herein are not limited to a linearpotential gradient, as any suitable potential gradient may be used. Insome embodiments, the electric field in the carrier travel/captureregion 106 may be high enough so charge carriers move fast enough in thecarrier travel/capture region 106 such that the transit time is smallcompared to the time over which photons may arrive. For example, in thefluorescence lifetime measurement context, the transit time of chargecarriers may be made small compared to the lifetime of a luminescentmarker being measured. The transit time can be decreased by producing asufficiently graded electric field in the carrier travel/capture region106.

FIG. 3C shows an embodiment in which the photon absorption/carriergeneration region 102 includes a PN junction. FIG. 3C shows an outerelectrode 302, which may be at a relatively low potential, thereby“pinning” the surface potential at a relatively low potential. Anelectrode 304 may be included to assist in producing the potentialgradient for a static electric field that drives carriers toward carriertravel/capture area 106 (the lower portion of carrier travel/capturearea 106 is not shown). FIG. 3C indicates regions of diffusion,polysilicon, contact and metal 1.

FIG. 3D shows a top view of a pixel as in FIG. 3C, with the addition ofdoping characteristics. FIG. 3D also shows the electric field sweepingcarriers down to region 106 along the potential gradient established bythe PN junction and the electrode 304. FIG. 3D indicates regions ofdiffusion, polysilicon, contact, metal 1, N-implant, P-implant, andP-epi.

FIG. 3E shows a top view of a pixel as in FIG. 3C, including the carriertravel/capture area 106.

FIG. 3F shows an array of pixels as in FIG. 3E. FIG. 3F indicatesregions of diffusion, polysilicon, contact and metal 1.

FIG. 3G shows the pixel array of FIG. 3F and also indicates regions ofdiffusion, polysilicon, contact, metal 1, N-implant, P-implant, andP-epi.

FIG. 4 shows a circuit diagram of the pixel 100A of FIG. 3B. The chargecarrier confinement area 103 is shown in heavy dark lines. Also shownare the electrodes, charge carrier storage area 108 and readoutcircuitry 110. In this embodiment, the charge storage bins bin0, bin1,bin2, and bin3 of carrier storage region 108 are within the carrierconfinement area 103 under electrode st1. As discussed above, in someembodiments a junction may be used to produce a static field in region102 instead of or in addition to the electrodes.

Light is received from a light source 120 at photon absorption/carriergeneration area 102. Light source 120 may be any type of light source,including a luminescent sample (e.g., linked to a nucleic acid) or aregion or scene to be imaged, by way of example and not limitation. Alight shield 121 prevents light from reaching carrier travel/capturearea 106. Light shield 121 may be formed of any suitable material, sucha metal layer of the integrated circuit, by way of example and notlimitation.

FIG. 5A illustrates a potential gradient that may be established in thecharge carrier confinement area 103 in photon absorption/carriergeneration area 102 and carrier travel/capture area 106 along the lineA-A′ of FIG. 3B. As illustrated in FIG. 5A, a charge carrier (e.g., anelectron) may be generated by absorption of a photon within the photonabsorption/carrier generation area 102. Electrodes Vb0-Vbn and b0-bm areset to voltages that increase to the right of FIG. 5A to establish thepotential gradient the causes electrons to flow to the right in FIG. 5A(the downward direction of FIG. 3B). Additionally or alternatively, a PNjunction may be present to establish or assist in establishing thefield. In such an embodiment, carriers may flow below the surface, andFIG. 5A (and related figures) shows the potential in the region wherethe carriers flow. Initially, carriers may be allowed to flow throughthe carrier travel/capture area 106 into the drain 104, as shown inFIGS. 6A, 6B and 6C. FIG. 6A shows the position of a carrier 101 once itis photogenerated. FIG. 6B shows the position of a carrier 101 shortlythereafter, as it travels in the downward direction in response to theestablished potential gradient. FIG. 6C shows the position of thecarrier 101 as it reaches the drain 104.

FIG. 5B shows that after a period of time a potential barrier 501 toelectrons may be raised at a time t1 by decreasing the voltage ofelectrode b0. The potential barrier 501 may stop an electron fromtraveling to the right in FIG. 5B, as shown in FIGS. 6D, 6E and 6F. FIG.6D shows the position of a carrier 101 (e.g., an electron) once it isphotogenerated. FIG. 6E shows the position of a carrier 101 shortlythereafter, as it travels in the downward direction in response to thepotential gradient. FIG. 6F shows the position of the carrier 101 as itreaches the potential barrier 501 after time t1.

FIG. 5C shows that after another time period, another potential barrier502 to electrons may be raised at time t2 by decreasing the voltage ofelectrode b2. If an electron arrives between electrodes b0 and b2between times t1 and t2, the electron will be captured between potentialbarrier 501 and potential barrier 502, as illustrated in FIG. 5C andFIG. 6G.

FIG. 5D shows that after another time period, another potential barrier503 to electrons may be raised at time t3 by decreasing the voltage ofelectrode b4. If an electron arrives between electrodes b2 and b4between times t2 and t3, the electron will be trapped in a locationbetween potential barrier 502 and potential barrier 503. In the exampleof FIGS. 5D and 6H, an electron arrived between times t1 and t2, so itremains captured between potential barrier 501 and potential barrier502.

FIG. 5E shows that after another time period, another potential barrier504 to electrons may be raised at time t4 by decreasing the voltage ofelectrode b6. If an electron arrives between electrodes b4 and b6between times t3 and t4, the electron will be trapped in a locationbetween potential barrier 503 and potential barrier 504. In the exampleof FIGS. 5E and 6I, an electron arrived between times t1 and t2, so itremains captured between potential barrier 501 and potential barrier502.

FIG. 5F shows that after another time period, another potential barrier505 to electrons may be raised at time t5 by decreasing the voltage ofelectrode bm. If an electron arrives between electrodes b6 and bmbetween times t4 and t5, the electron will be trapped in a locationbetween potential barrier 504 and potential barrier 505. In the exampleof FIGS. 5F and 6J, an electron arrived between times t1 and t2, so itremains captured between potential barrier 501 and potential barrier502.

FIG. 6K shows a voltage timing diagram illustrating the voltages ofelectrodes b0-b8, st0 and st1 over time. A charge carrier moving throughthe carrier travel/capture area 106 during the sequence of raisingpotential barriers 501-505 will be captured at a location within thecarrier travel/capture area 106 that depends on the time at which itarrives at the carrier travel/capture area 106, which in turn dependsupon the time at which the charge carrier was generated by photonabsorption in photon absorption/carrier generation area 102. The timingwith which potential barriers 501-505 are raised sets the timing of thecharge storage bins bin0-bin3. As shown in FIG. 6K, a carrier thatarrives between times t1 and t2 will be trapped within a time intervalfor bin0, a carrier that arrives between times t2 and t3 will be trappedwithin a time interval for bin1, a carrier that arrives between times t3and t4 will be trapped within a time interval for bin2, and a carrierthat arrives between times t4 and t5 will be trapped within a timeinterval for bin3.

After the sequence shown in FIG. 5A-5F, a captured charge carrier maythen be transferred to the appropriate charge carrier storage bin basedon the location at which the charge carrier is captured within thecarrier travel/capture area 106. In this embodiment, if an electron iscaptured under electrode b1, it is transferred to bin0. If an electronis captured under electrode b3, it is transferred to bin1. If anelectron is captured under electrode b5, it is transferred to bin2. Ifan electron is captured under electrode b7, it is transferred to bin3.In some embodiments, transfer of any captured carrier(s) within thecarrier travel/capture area 106 to their corresponding bin(s) may beperformed in parallel (e.g., simultaneously). However, the techniquesdescribed herein are not limited as to transferring captured carriers tocharge storage bins in parallel.

As shown in FIG. 6K, after the sequence shown in FIG. 5A-5F the voltageson electrodes st0 and st1 may be changed to transfer any captured chargecarriers to the corresponding charge carrier storage bin(s). An examplesequence for transferring captured charge carrier(s) will be discussedwith respect to FIG. 6K and FIGS. 7A-7G.

FIG. 7A shows a plot of the of potential for a cross section of thecharge carrier confinement area 103 along the line B-B′ of FIG. 3B. FIG.7A shows the potential at time t5 (FIG. 6K) in an example where anelectron is captured between potential barriers 503 and 504. A plan viewshowing an electron captured between potential barriers 503 and 504 isshown in FIG. 7E.

FIG. 7B shows that after time t5 the voltage on electrodes b1, b3, b5and b7 optionally may be decreased (not shown in FIG. 6K) to raise theposition of an electron within the potential well, to facilitatetransferring the electron.

FIG. 7C shows that at time t6 (FIG. 6K), the voltages on electrodes st0and st1 may be raised. Changing the voltages of the electrodes in thismanner may provide a potential gradient that causes a transfer a chargecarrier captured in carrier travel/capture area 106 to a correspondingcharge storage bin under electrode st1. A plan view showing the voltageof electrode st1 being raised and the carrier 101 being transferred isshown in FIG. 7F.

FIG. 7D shows that at time t7, the voltage on electrode st0 may bedropped, thereby confining the captured carrier (if any) in thecorresponding bin (bin2 in this example). The voltage on electrode b6may be raised at time t8 to reestablish the potential gradient in thecarrier travel/capture area 106. A plan view showing the voltageelectrode st1 being lowered and the carrier 101 being captured in bin2is shown in FIG. 7G.

FIG. 7H shows the characteristics of the electrodes of a charge carriersegregation structure, according to some embodiments. FIG. 7H specifies,for each electrode, the voltage during the gradient phase, the voltageduring the binning phase, the voltage during the transfer phase, thevoltage during the readout phase the high, and type of voltage change.However, this is merely an example, and the techniques described hereinare not limited as to the implementation details illustrated in FIG. 7H.

Example Sequence of Measurements

Repeating the process of photon absorption/carrier generation and timebinning of photogenerated charge carriers may enable gatheringstatistical information about the times at which photons arrive at thephotodetector, as discussed below.

In some embodiments, a “measurement” may include receiving a photon,capturing a charge carrier at a particular time and/or location andtransferring the captured carrier to a charge storage node correspondingto a particular time period or bin. A measurement may be repeated aplurality of times to gather statistical information about the times atwhich photons arrive at the photodetector.

FIG. 8A shows a flowchart of a method 700 that includes performing aplurality of measurements 720, according to some embodiments. Such amethod may be performed at least partially by an integrated device asdescribed herein.

In step 702 a measurement 720 may be initiated by a trigger event. Atrigger event may be an event that serves as a time reference for timebinning arrival of a photon. The trigger event could be an optical pulseor an electrical pulse, for example, and could be a singular event or arepeating, periodic event. In the context of fluorescence lifetimemeasurement, the trigger event may be the generation of a lightexcitation pulse to excite a fluorophore. In the context oftime-of-flight imaging, the trigger event may be a pulse of light (e.g.,from a flash) emitted by an imaging device comprising the integratedphotodetector. The trigger event can be any event used as a referencefor timing the arrival of photons or carriers.

The generation of the light excitation pulse may produce a significantnumber of photons, some of which may reach the pixel 100 and may producecharge carriers in the photon absorption/carrier generation area 102.Since photogenerated carriers from the light excitation pulse are notdesired to be measured, they may be allowed to flow down the electricpotential to the drain 104 without being captured. Allowingphotogenerated carriers produced by a light excitation pulse to flow tothe drain 104 without being captured may reduce the amount of unwantedsignal that otherwise may need to be prevented from arriving by complexoptical components, such as a shutter or filter, which may addadditional design complexity and/or cost. The timing of the raising ofone or more potential barriers within the carrier travel/capture area106 may be timed such that photogenerated carriers caused by anyunwanted optical signal flow to the drain 104. Moreover, this techniquemay be used with any number of time bins, including embodiments withonly a single time bin. For example, a pixel may include a single timebin and a drain where the timing of the potential barriers reducessignal associated with the excitation pulse while capturing the desiredoptical signal within the carrier travel/capture area 106.

The measurement 720 may then commence at step 704, in which photon(s)desired to be detected may be absorbed and a charge carrier may begenerated in region 102. In the context of fluorescence lifetimemeasurement or time-of-flight imaging, step 704 may commence after thelight excitation pulse is completed.

In step 706 charge carrier(s) moving through the carrier travel/capturearea 106 may be captured at predetermined locations at selected timeswith respect to trigger event 702. In some embodiments, chargecarrier(s) may be captured in one or more regions of the carriertravel/capture area 106 by raising one or more potential barriers totrap a carrier in a location that depends upon the time at which it wasgenerated by photon absorption, as discussed above.

In step 708 captured charge carrier(s), if present, may be transferredfrom the location at which captured charge carrier(s) were captured to acorresponding charge storage bin, thereby “time-binning” the chargecarrier.

Following step 708 the measurement 720 may be repeated n−1 times toobtain statistical information regarding the time periods at whichphotons tend to arrive after a trigger event 702. Time-binned chargecarriers may be aggregated in the corresponding charge storage bins asthe measurement 720 is repeated. Repeating the measurement 720 mayenable aggregating a sufficient number of charge carriers in the chargecarrier storage bins to provide statistically meaningful results. Forexample, in the context of fluorescence lifetime measurement, it may beexpected that a photon absorption event in response to a photon receivedfrom a fluorophore may occur relatively rarely. For example, such anevent may be expected to occur once in about 1,000 measurements.Accordingly, a large number of measurements 720 may need to be performedto aggregate a sufficient number of charge carriers in the chargecarrier storage bins such that the results are statistically meaningful.In some embodiments, the number of measurements n of a fluorophore thatmay be performed for fluorescence lifetime measurement may be 500,000 ormore, or 1,000,000 or more, to enable capturing and binning a sufficientnumber of charge carriers in each bin (i.e., tens or hundreds, or more,in some embodiments).

Once the allotted number of measurements n has been performed, themethod 700 may proceed to step 710 of reading out the time bins. Readingout the time bins may include converting the amount of charge aggregatedin each of the charge storage bins into corresponding voltages, as willbe discussed below.

FIG. 8B is a diagram showing an excitation pulse being generated at timet0, and time bins bin0-bin3. Note that in this example the time bins formeasuring photons do not begin until t1, a period of time after t0,which lets the excitation light end prior to measuring signal photons.

FIG. 8C shows a plot of the number of photons/charge carriers in eachtime bin for a set of fluorescence lifetime measurements in which theprobability of a marker or die fluorescing decreases exponentially overtime. By repeating the sequence of excitation, charge capture, andtransfer into respective bins many times, and reading out the quantityof charge carriers transferred into each bin, a histogram of the numberof photons registered in different bins may be produced that allowsdetermining or approximating the lifetime of a fluorophore.

Method 700 may be performed over any suitable time period in whichphotons are desired to be captured. In the context of fluorescencelifetime measurement, a suitable period for performing method 700 may be10 milliseconds, for example. In some embodiments, steps 702 to 708 maybe repeated at a frequency that is the MHz range. In some embodiments,the time bins may have a resolution on the scale of picoseconds ornanoseconds.

Temporal Multiplexing of Detection in Response to Different TriggerEvents

In some embodiments, measurements may be performed using a plurality ofdifferent types of trigger events. The trigger events may be multiplexedin time such that a pixel receives light in response to different typestrigger events in different time periods. For example, in the context ofluminance lifetime measurements, the trigger events may be excitationlight pulses (e.g., laser pulses) of different wavelengths λ1 and λ2,which can excite different luminescent molecules (e.g., fluorophores).In some embodiments, fluorophores may be identified and/or discriminatedfrom one another based on their response to different wavelengths λ₁ andλ₂ of excitation light. Exciting a sample with light excitation pulsesof wavelengths λ₁ and λ₂ at different times, and analyzing the luminanceemitted by the sample in response, can enable detecting and/oridentifying luminescent molecules based on whether luminescence isdetected in a first time period in response to excitation light ofwavelength λ₁, or in a second time period in response to excitationlight of wavelength λ₂. In addition to, or as an alternative to suchtemporal multiplexing, luminescent molecules may be identified and/ordiscriminated based upon measuring their luminance lifetimes.

In some embodiments, a nucleic acid may be sequenced based upondetecting light emitted by one or more fluorophores attached tonucleotides of the nucleic acid. In some embodiments, such sequencingmay be performed by temporal multiplexing of excitation light ofdifferent wavelengths, based upon measuring luminance lifetimes, orbased upon a combination of such techniques.

For example, in some embodiments, four different fluorophores may belinked to respective nucleotides (e.g., A, C, G and T) of a nucleicacid. The four fluorophores may be distinguishable from one anotherbased upon a combination of excitation wavelength and luminancelifetime, as illustrated in the chart below.

λ₁ λ₂ Short Lifetime Fluorophore 1 Fluorophore 3 Long LifetimeFluorophore 2 Fluorophore 4

In some embodiments, the integrated photodetector may temporallymultiplex detection of photons produced by a sample in response to lightexcitation pulses of different wavelengths. For example, in a first timeperiod, light produced by a sample in response to excitation light ofwavelength λ₁ may be detected. Subsequently, in a second time period,light produced by a sample in response to excitation light of wavelengthλ₂ may be detected. To do so, a pixel having a plurality of time binsmay use a first subset of time bins to detect arrival of photons in thefirst time period and a second subset of time bins to detect arrival ofphotons in the second time period. By examining whether light arrives ata pixel during the first time period or the second time period, it canbe determined whether a fluorophore is fluorescing in response to lightof wavelength λ₁ or light of wavelength λ₂.

In some embodiments, information regarding the arrival times of photonsin response to a light excitation pulse can be used to determine and/ordiscriminate fluorescence lifetime, and thereby identify a fluorophore.In some embodiments, a first excitation pulse of a first wavelength maybe emitted, then a first subset of the time bins of a pixel may be usedto time-bin the arrival of incident photons in a first time interval.Then, a second excitation pulse of a second wavelength may be emitted,and a second subset of time bins of the pixel may be used to time-binthe arrival of incident photons in a second time interval. Accordingly,if photons are received in the first time interval and/or the secondtime interval, information about the lifetime of the fluorophore thatproduced the photons can be obtained. Repeating the process of temporalmultiplexing of light excitation pulses along with measuring informationregarding fluorescence lifetimes can provide sufficient information toenable identification of the fluorophore. Accordingly, the nucleotide towhich the fluorophore is attached may be identified. As a sequencingreaction progresses, additional nucleotides may be incorporated into apolymerase over time. Performing and repeating the process of temporalmultiplexing of light excitation pulses with measurements offluorescence lifetimes can provide sufficient information to enableidentification of such fluorophores. Accordingly, the sequence ofnucleotides in a nucleic acid can be determined.

FIG. 8D shows a method of operating the integrated photodetectoraccording to some embodiments in which light is received at theintegrated photodetector in response to a plurality of different triggerevents. FIG. 8E illustrates voltages of the electrodes of the chargecarrier segregation structure when performing the method of FIG. 8D.

In step 802, a measurement 820 may be initiated by a trigger event A.Trigger event A may be an event that serves as a time reference for timebinning arrival of a photon. The trigger event may be an optical pulseor an electrical pulse, for example, and could be a singular event or arepeating, periodic event. In the context of fluorescence lifetimemeasurement, the trigger event A may be the generation of a lightexcitation pulse at a first wavelength to excite a first type offluorophore.

The generation of the light excitation pulse may produce a significantnumber of photons, some of which may reach the pixel 100 and may producecharge carriers in the photon absorption/carrier generation area 102.Since photogenerated carriers from the light excitation pulse are notdesired to be measured, they may be allowed to flow down the electricpotential to the drain 104 without being captured, as discussed above.The raising of one or more potential barriers within the carriertravel/capture area 106 may be timed such that photogenerated carrierscaused by any unwanted optical signal flow to the drain 104.

The measurement 820 may then proceed at step 804, in which photon(s)desired to be detected may be absorbed and a charge carrier may begenerated in region 102. In the context of fluorescence lifetimemeasurement, step 804 may commence after the light excitation pulse iscompleted.

In step 806, charge carrier(s) moving through the carrier travel/capturearea 106 may be captured at predetermined locations at selected timeswith respect to trigger event 802. In some embodiments, chargecarrier(s) may be captured in one or more regions of the carriertravel/capture area 106 by raising one or more potential barriers totrap a carrier in a location that depends upon the time at which it wasgenerated by photon absorption, as discussed above. In some embodiments,step 806 may include raising potential barriers 501, 503 and 503 insuccession, thereby capturing charge (if present) corresponding to timebins bin0 and/or bin1.

In step 808, captured charge carrier(s), if present, may be transferredfrom the location at which they were captured to a corresponding chargestorage bin, thereby “time-binning” the charge carrier. For example, anycharge captured corresponding to time bins bin0 and/or bin1 may betransferred to bins bin0 and/or bin1 in step 808 using a technique shownin FIGS. 7A-7D, for example.

In step 810, a second measurement 821 may be initiated by a triggerevent B. Trigger event B may be an event that serves as a time referencefor time binning arrival of a photon. The trigger event may be anoptical pulse or an electrical pulse, for example, and could be asingular event or a repeating, periodic event. In the context offluorescence lifetime measurement, the trigger event B may be thegeneration of a light excitation pulse at a second wavelength to excitea second type of fluorophore.

The generation of the light excitation pulse may produce a significantnumber of photons, some of which may reach the pixel 100 and may producecharge carriers in the photon absorption/carrier generation area 102.Since photogenerated carriers from the light excitation pulse are notdesired to be measured, they may be allowed to flow down the electricpotential to the drain 104 without being captured, as discussed above.The raising of one or more potential barriers within the carriertravel/capture area 106 may be timed such that photogenerated carrierscaused by any unwanted optical signal flow to the drain 104.

The second measurement 821 may then proceed at step 812, in whichphoton(s) desired to be detected may be absorbed and a charge carriermay be generated in region 102. In the context of fluorescence lifetimemeasurement, step 812 may commence after the second light excitationpulse is completed.

In step 814, charge carrier(s) moving through the carrier travel/capturearea 106 may be captured at predetermined locations at selected timeswith respect to trigger event 810. In some embodiments, chargecarrier(s) may be captured in one or more regions of the carriertravel/capture area 106 by raising one or more potential barriers totrap a carrier in a location that depends upon the time at which it wasgenerated by photon absorption, as discussed above. In some embodiments,step 814 may include raising potential barriers 503, 504 and 505 insuccession, thereby capturing charge (if present) corresponding to timebins bin2 and/or bin3.

In step 816, captured charge carrier(s), if present, may be transferredfrom the location at which they were captured to a corresponding chargestorage bin, thereby “time-binning” the charge carrier. For example, anycharge captured corresponding to time bins bin2 and/or bin3 may betransferred to bins bin2 and/or bin3 in step 816 using a technique shownin FIGS. 7A-7D, for example.

Although an example has been described in which a pixel has four timebins, and two bins are allocated to measuring arrival times of lightproduced in response to each of the respective light excitation pulses,the techniques described herein are not limited in this respect. Forexample, the pixel may have a larger or smaller number of bins, whichmay be allocated in any suitable way to measuring light in response todifferent excitation pulses. Further, the techniques described hereinare not limited to light excitation pulses of two different wavelengths,as light excitation pulses of any number of wavelengths may be used, andmultiplexed accordingly.

Following step 816, the measurements 820 and 821 may be repeated n−1times to obtain statistical information regarding the time periods atwhich photons tend to arrive after a trigger event. Time-binned chargecarriers may be aggregated in the corresponding charge storage bins asthe measurements are repeated.

Once the allotted number of measurements n has been performed, themethod 800 may proceed to step 710 of reading out the time bins. Readingout the time bins may include converting the amount of charge aggregatedin each of the charge storage bins into corresponding voltages, as willbe discussed below.

Example Readout Circuitry and Sequences

As illustrated in FIGS. 2A and 2B, pixel 100 may include readoutcircuitry 110 that allows reading out the charge stored in the chargestorage bin(s) of the charge carrier storage region 108. Pixel 100 maybe an active pixel, such that readout circuitry 110 includes a readoutamplifier, or a passive pixel in which readout circuitry 110 does notinclude a readout amplifier. Any suitable type of active pixel orpassive pixel readout circuitry may be used.

If readout circuitry 110 includes a readout amplifier, any suitable typeof amplifier may be used. Examples of suitable amplifiers includeamplifiers abased on a common source configuration and amplifiers abasedon a source-follower configuration. However, the techniques describedherein are not limited as to any particular amplifier configuration.

If readout circuitry 110 includes a readout amplifier, the readoutamplifier may take the charge accumulated in a charge storage bin (e.g.,bin0, bin1, bin2 or bin3) as an input and produce a voltagerepresentative of the charge in the charge storage bin as an output.

One example of readout circuitry 110 based on a source-followerconfiguration is illustrated in FIG. 4. The example of readout circuitry110 shown in FIG. 4 is a “4T” configuration having four transistors: rt,sf, rs, and one of the transfer gates tx0-tx3. Since the threetransistors rt, sf, and rs are shared among each charge storage bin, theexample circuitry shown in FIG. 4 for all four bins is a “1.75T”configuration, (4 transfer gates+3 transistors)/4 bins. However, thetechniques described herein are not limited to using readout circuitry110 having a 1.75T configuration, as any other suitable type of readoutconfiguration may be used.

Further, any suitable readout techniques may be used, including noisereduction techniques. In some embodiments, readout circuitry 110 mayread out the charge carrier storage bins using correlated doublesampling. Correlated double sampling is technique in which a firstsample may be taken of a node at a reset voltage level which includes anundetermined amount of noise, and a second sample may be taken of asignal level at the node including the same undetermined noise. Thenoise can be subtracted out by subtracting the sampled reset level fromthe sampled signal level.

Readout circuitry 110 may perform readout of the charge storage binssequentially or in parallel. An example of a timing diagram forsequentially reading out bins bin0-bin3 with readout circuitry 110 shownin FIG. 4 using correlated double sampling is shown in FIG. 9A. As shownin FIG. 9A, initially reset transistor rt may be turned on to set thefloating diffusion node fd to a reset voltage ct. During the time periodin which the voltage of the floating diffusion node is reset thetransfer gates tx0-tx3 are turned off to keep the charge carriers storedin their respective bins. After the floating diffusion node fd is resetthe reset voltage may be sampled by turning off transistor rt andturning on transistor rs to produce an output voltage cb. The resetvoltage represented by output voltage cb may be stored in an analogformat (e.g., on a capacitor) or in a digital format (e.g., by A/Dconversion and storage). Then, transfer gate tx0 may be turned on toallow the charge from bin0 to flow to the floating diffusion fd. Thesignal voltage may be sampled by turning on transistor rs to produce anoutput voltage cb based on the charge stored in bin0. The signal voltagerepresented by output voltage cb may be stored in an analog format(e.g., on a capacitor) or in a digital format (e.g., by A/D conversionand storage).

Then, transistor rt may be turned on to set the floating diffusion fd toa reset voltage ct. During the time period in which the voltage of thefloating diffusion node fd is reset the transfer gates tx0-tx3 areturned off to keep the charge carriers stored in their respective bins.After the floating diffusion node fd is reset the reset voltage may besampled by turning off transistor rt and turning on transistor rs toproduce an output voltage cb. Again, the reset voltage represented byoutput voltage cb may be stored in an analog format (e.g., on acapacitor) or in a digital format (e.g., by A/D conversion and storage).Then, transfer gate tx1 may be turned on to allow the charge from bin1to flow to the floating diffusion. The signal voltage may be sampled byturning on transistor rs to produce an output voltage cb based on thecharge stored in bin1. Again, the signal voltage represented by outputvoltage cb may be stored in an analog format (e.g., on a capacitor) orin a digital format (e.g., by A/D conversion and storage).

The same process may then be performed for bin2 and bin3 by performing areset, sampling the reset voltage, transferring the charge from a bin tothe floating diffusion node fd, and sampling the signal. Accordingly, inthe readout sequence illustrated in FIG. 9A, eight samples may be takenrepresenting the reset value and signal values for the four bins. Thestored reset value for each bin may be subtracted from the stored signalvalue to obtain a result indicative of the charge stored in each bin,thus completing the correlated double sampling process.

Optionally, as discussed above, the sampled reset voltage level for abin may be stored on a first capacitor and the sampled signal for thebin may be stored on a second capacitor. Optionally, before sampling thereset level and signal level onto the capacitors the capacitors may becleared by setting them to the same voltage.

FIG. 9B shows a readout sequence for performing correlated doublesampling that does not require measuring a reset value for each signalvalue, according to some embodiments. In the example of FIG. 9B, asingle reset value is measured for all the bins of the pixel. To obtainthe signal for the first bin, a reset value may be subtracted from themeasured signal value, as discussed above. Instead of resetting thefloating diffusion at this point, charge may be transferred to thefloating diffusion from the second bin, thereby aggregating the chargefor the first and second bins. The signal for the second bin can beobtained by subtracting the signal for the first bin from the aggregatedsignal for the first and second bins. Since both the signal for thefirst bin and the aggregated signal for the first and second binsinclude the same reset noise, the result is that the reset noise issubtracted out. The process may continue for the remaining bins, withthe aggregated signal for the previous bin being subtracted from theaggregated signal for the next bin. Aggregating the stored charge forthe bins in this manner can allow reading our larger signals than wouldbe the case if each bin were read out individually, and can reducenoise, as the sampled signals will be higher above the noise floor thanwould be the case if each bin were read out individually. In the examplewith four time bins, five samples may be taken—one reset value and foursamples representing the cumulative charge stored in the charge storagebins. This process will be described in greater detail with reference toFIG. 9B.

As shown in FIG. 9B, initially reset transistor rt may be turned on toset the floating diffusion node fd to a reset voltage ct. During thetime period in which the voltage of the floating diffusion node is resetthe transfer gates tx0-tx3 are turned off to keep the charge carriersstored in their respective bins. After the floating diffusion node fd isreset the reset voltage may be sampled by turning off transistor rt andturning on transistor rs to produce an output voltage cb. The resetvoltage represented by output voltage cb may be stored in an analogformat (e.g., on a capacitor) or in a digital format (e.g., by A/Dconversion and storage). Then, transfer gate tx0 may be turned on toallow the charge from bin0 to flow to the floating diffusion. The signalvoltage for bin0 may be sampled by turning on transistor rs to producean output voltage cb based on the charge stored in bin0.

Then, transfer gate tx1 may be turned on to allow the charge from bin1to flow to the floating diffusion. The signal voltage for bin1+bin0 maybe sampled by turning on transistor rs to produce an output voltage cbbased on the charge stored in bin1 plus the charge stored on bin0. Theoutput signal value for bin0 may be subtracted from the output signalvalue for bin0+bin1 to produce a signal indicative of the charge storedon bin1.

A similar process may then be performed for bin2 and bin3 by subtractingthe measured signal level for bin n from the measured signal level forbin n+1. Accordingly, using such a technique the number of samples thatmay need to be taken may be reduced.

The following formulas show how to calculate the “corrected” (usingcorrelated double sampling) signal for each bin using only a singlemeasured reset value.

corrected signal bin0=measured signal bin0−reset level

corrected signal bin1=measured signal for(bin0+bin1)−measured signalbin0

corrected signal bin2=measured signal for(bin0+bin1+bin2)−measuredsignal for(bin0+bin1)

corrected signal bin3=measured signal for(bin0+bin1+bin2+bin3)−measuredsignal for(bin0+bin1+bin2)

In some embodiments, oversampling of the readout from a pixel may beperformed. Oversampling involves reading the same signal from the pixela plurality of times. Each time a signal is read from the pixel, theremay be slight variations in the signal that is read due to noise.Oversampling of the readout of a signal and averaging the samples canreduce the noise (e.g., white noise) in measurements. In someembodiments, multiple samples may be taken (e.g., 4-8 samples) to read asingle nominal signal value from the pixel (e.g., a single reset levelor signal level). In some embodiments, each of the samples of a signalmay be read out through the readout signal change and converted intodigital values (e.g., digital words). The average of the samples maythen be calculated, and the average used as the measured signal from thepixel. For example, if oversampling by 8× is used, eight samples may betaken for each reset and signal value, for a total of 64 samples in thecase of measuring 4 time bins and 4 reset levels, or 40 samples in thecase of measuring 1 reset level and 4 aggregated signal levels.

Pixel Array Readout Circuitry

Readout in Parallel, Sequential Readout, and Readout with a Combinationof Parallel and Sequential Readout

As discussed above, the pixel array may include a plurality of pixelsarranged in rows and columns. In some embodiments, readout may beperformed row by row. In some embodiments, a row of the pixel array maybe selected, and a readout process may be performed for the selected rowof pixels. The readout circuitry for a column of pixels may be common tothe pixels in the column, such that readout may be performed by thereadout circuitry for respective pixels in the column as different rowsare selected. Readout for a selected row may be performed in parallel(termed “column parallel”), sequentially, or a combination of paralleland sequentially (termed “semi-column parallel”).

To perform readout of the pixels of a selected row in column parallel,individual readout circuitry may be provided for each column so that thepixels of each column in the selected row can be read out at the sametime, as illustrated in FIG. 10A. FIG. 10A illustrates an array ofpixels having a plurality of columns C1 to Cn and a plurality of rows,with a selected row Ri being shown by way of illustration. In theembodiment of FIG. 10A, each column of pixels has an associated readoutcircuit 905. Since each column of pixels has an associated readoutcircuit 905, the signals from each pixel in row Ri can be read out atthe same time.

To perform readout of the pixels of a selected row in sequence,individual readout circuitry need not be provided for each column. Forexample, in some embodiments a common readout circuit may be provided,and each pixel of the selected row may be read out sequentially. FIG.10B shows an embodiment in which a common readout circuit 905 may beprovided for a plurality of columns. The common readout circuit may beselectively connected to a column by a switch network 906 under thecontrol of suitable control circuitry. For example, in some embodiments,switch network 906 may be sequentially connect individual columns ofpixels to the readout circuit 905.

To perform readout of the pixels in semi-column parallel, a plurality ofreadout circuits 905 may be provided, fewer than the number of columns,as illustrated in FIG. 10C. In such a semi-column parallel architecture,each readout circuit 905 may be shared by a subset of the columns. Eachreadout circuit 905 may sequentially read out a subset of columns in thearray. As shown in FIG. 10C, readout circuit 905A may be selectivelyconnected to its respective columns by a switch network 906A. Readoutcircuit 905B may be selectively connected to its respective columns by aswitch network 906B.

In some embodiments, a readout circuit 905 may include one or moreamplifier(s) to amplify a signal from a pixel and an analog to digitalconverter to convert the amplified signal into a digital value. Examplesof configurations of readout circuits 905 according to variousembodiments are described below.

Sample and Hold Circuit

In some embodiments, the readout circuitry for a column may include oneor more sample and hold circuits. FIG. 10D shows a circuit diagramillustrating column readout circuitry 905C, which includes sample andhold circuitry 907, amplifier circuitry 901, and an analog-to-digital(A/D) converter 902. The sample and hold circuit 907 may sample theoutput voltage from a pixel (e.g., at node cb) onto a capacitive element(e.g., a capacitor), and then hold the voltage on the capacitor while itis read out by an amplifier. As discussed above, the output voltage fromthe pixel may represent the number of charge carriers captured duringone or more time intervals.

The sample and hold circuit may operate in a plurality of phases, termeda “sample” phase and a “hold” phase. In the “sample” phase, the voltagevalue from the pixel may be sampled onto a capacitive element. Thevoltage to be read out is thus stored on the capacitive element.Following the “sample” phase, the voltage of the capacitor is read inthe “hold” phase. During the “hold” phase, the voltage of the capacitormay be read out from the capacitive element and processed by one or moreamplifiers and then converted into digital form by an analog to digital(A/D) converter. As illustrated in FIG. 10D, during the sample phase(φ1), switch s1 is turned on (set in its conductive state) and switch s2is turned off (set in its non-conductive state), thereby sampling thevoltage from readout terminal cb of a pixel onto a capacitive element,e.g., capacitor C1. The hold phase (φ2) follows the sample phase. Duringthe hold phase the switch s1 is turned off and the switch s2 is turnedon, thereby connecting the capacitor C1 to the amplifier circuitry 901.By turning off switch S 1, the voltage of the capacitor may be heldsubstantially constant while the voltage is read, as the amplifiercircuitry 901 may have a high input impedance. The amplified signal fromthe amplifier circuitry 901 may be provided to an A/D converter 902 toconvert the amplified voltage into a digital value.

In some embodiments, power consumption and/or cost can be reduced byreducing or minimizing the number of circuits (e.g., amplifiers, analogto digital converters) used. In some embodiments, to reduce or minimizethe number of circuits in the readout chain one or more circuits of thereadout chain may be shared by more than one column of the pixel array.

Multiplexing Readout Circuitry Component(s)

In some embodiments, one or more components of the readout circuitry maybe shared by two or more columns of the pixel array. For example, asshown in FIG. 10E, all or a portion of amplifier circuitry 901, the A/Dconverter 902, or both, may be shared by two or more columns of thepixel array. FIG. 10E illustrates an embodiment of readout circuitry905D in which both the amplifier circuitry 901 and the A/D converter 902are shared by two columns of the pixel array. In the embodiment of FIG.10E, respective column lines are connected to respective pixel nodes cb1and cb2. Each column line is connected to a respective sample and holdcircuit 907A, 907B. Amplifier circuitry 901 and A/D converter 902 may beshared by both columns. The input to the amplifier circuitry 901 may bemultiplexed between the sample and hold circuits 907A and 907B such thattheir outputs are connected to the amplifier circuitry 901 at differenttimes (e.g., sequentially). By using shared readout circuit componentssuch as amplifier circuitry 901 and/or A/D converter 902, the number ofcomponents in the readout circuitry can be reduced, which can reduce thecost and/or power consumption of the readout circuitry.

In some embodiments, the sample and hold phases for the columns sharingthe amplifier circuitry 901 may be alternated, such that when a thecolumn is in the sampling phase and not connected to the amplifiercircuitry 901, the other column is in the hold phase and its sample andhold circuit is connected to amplifier circuitry 901 to amplify thevoltage it previously sampled. In the embodiment of FIG. 10F, the sampleand read phases are alternated between the two columns, with the uppercolumn being in the sample phase during phase 1 and in the hold phaseduring phase 2, and the lower column being in the sample phase duringphase 2 and the hold phase during phase 1. During phase 1 (φ1), thesignal from node cb1 is sampled onto capacitor C1 by turning on switchs1, and switch s2 is turned off, switch s3 is turned off, and capacitorC2 is connected to the amplifier 901 via switch s4, which is turned on.During phase 2 (φ2), the signal from node cb 2 is sampled onto capacitorC2 by turning on switch s3, switch s4 is turned off, switch s1 is turnedoff, and capacitor C1 is connected to the amplifier 901 via switch s2,which is turned on. Sharing the amplifier circuitry 901 by more than onecolumn may reduce the downtime of amplifier circuitry 901, as it doesnot need to sit idle during a sampling phase for a column.

In some embodiments, more than two columns of the pixel array may sharereadout circuitry 901 and/or A/D converter 902. FIG. 10F shows anembodiment in which n columns of the pixel array share readout circuitry901 and/or A/D converter 902. Capacitors C1-Cn may be sequentiallyconnected to the readout circuitry 901 to read out their voltage values.Capacitors C1-Cn may be connected to the readout circuitry 901 in anysuitable order. The sampling phase of the respective sample and holdcircuits for each column may be timed to occur during a period in whichthe sample and hold circuit is not being read out by the amplifiercircuitry 901. In some embodiments, and as discussed above, the samplingphases may be timed to occur during a time interval in which theamplifier circuitry 901 is reading out a different row, to limit theamount of time the amplifier circuitry 901 sits idle. For example, asdiscussed above, the voltage from node cb1 may be sampled on capacitorC1 during phase 1. During phase 2, the voltage of capacitor C1 may beread out by amplifier circuitry 901 and the voltage from node cb2 may besampled on capacitor C2. During phase 3, the voltage of capacitor C2 maybe read out by amplifier circuitry 901 and the voltage from a third nodecb3 may be sampled on a third capacitor C3, etc. The process may thenbegin again with phase 1 starting during the time the last column (rown) is read out by amplifier circuitry 901, or after the last column isread out by amplifier circuitry 901. Any suitable number of columns mayshare amplifier circuitry 901, such as 2, 4, 8, 16, 32, 64, 128, etc.,or any other suitable number (which need not be a power of 2).

FIG. 10G shows a diagram of readout circuitry including amplifiercircuitry 901. In the embodiment of FIG. 10G, amplifier circuitry 901includes a plurality of amplifiers 910 and 911. Using a plurality ofcascaded amplifiers 910 and 911 can reduce power consumption, asachieving the desired signal gain may be achieved with less powerdissipation when a plurality of amplifiers 910 and 911 are used asopposed to using a single amplifier to achieve the same gain.

FIG. 10H shows a diagram of readout circuitry including amplifiercircuitry 901 having first stage amplifiers 910A and 910B for respectivecolumns and a second stage amplifier 911 that is shared by the twocolumns. A multiplexer 912 connects first stage amplifiers 910A and 910Bto the second stage amplifier 911 at different times. In someembodiments, the amplifiers 910A, 910B and 911 may be differentialamplifiers.

FIG. 10I shows a diagram of readout circuitry including first-stageamplifiers 910A and 910B, a second stage amplifier 911 and a third stageamplifier 912. As discussed above, using an additional amplifier stageto achieve a desired gain value may reduce power consumption withrespect to using fewer amplifier stages to achieve the desired gainvalue. In some embodiments, the amplifiers 910A, 910B, 911 and 912 maybe differential amplifiers.

In some embodiments, gain may be applied in the signal chain in aplurality of stages. In some embodiments, the first-stage amplifier(e.g., 910A, 910B) may have a gain of 2 or more, the second stageamplifier (e.g., 911) may have a gain of 1-8, or more, and the thirdstage amplifier (e.g., 912) may have a gain of 1-2, or more, for anoverall gain of the three stages of 2-32, or more.

In some embodiments, the amplifiers may have a digitally programmablegain. The gain of one or more stages may be changed depending on thecharacteristics of the light being received. For example, if more thanone wavelength of light excitation pulse (e.g., laser pulse) is usedthat produce different responses in the pixel, the gain of one or moreamplifiers in the readout chain may be changed depending on whichwavelength of light is currently being detected. If one wavelengthresults in smaller number of charge carriers being produced, the gainmay be increased to accommodate the reduced signal level. If anotherwavelength results in a larger number of charge carriers being produced,the gain may be decreased. In some embodiments, the gains of the readoutchain for different wavelengths may be normalized to one another toproduce the same output levels in response to different wavelengths.

Readout Circuitry Design Considerations

Since in some embodiments, the number of charge carriers captured foreach time bin may be relatively small, e.g., on the order of hundreds ofcharge carriers, the signal to be detected from each pixel may berelatively small. Accordingly, in some embodiments the signal chainrunning from a pixel to (and including) an analog to digital convertermay include low-noise readout circuitry. Techniques and circuits forlimiting the noise in the readout chain will be discussed below.

In some embodiments, differential processing of signals may reduce orminimize noise in the readout chain. Differential processing of signalscan reject common-mode noise that may be injected into the readoutchain. The readout circuitry may include one or more differentialcomponents, such as a differential sample and hold circuit, differentialamplifier(s) and/or a differential A/D converter. In some embodiments,differential signal processing may be used as early as possible in thereadout chain (e.g., as close as possible to the pixel output), to avoidinjecting common-mode noise into the readout chain. In some embodiments,the entire readout chain from a pixel output to a digital word may beperformed by differential circuit components. However, the techniquesdescribed herein are not limited in this respect, as in some embodimentsone or more single-ended readout circuitry components may be used.

FIG. 10J shows readout circuitry shared by two columns including adifferential sample and hold circuit 908 and a differential amplifier909. The differential sample and hold circuit 908 includes capacitorsCin1 for a first column of the pixel array and capacitors Cin2 for asecond column of the pixel array. The differential amplifier 909includes capacitors Cf1 for a first column of the pixel array andcapacitors Cf2 for a second column of the pixel array.

FIG. 10K shows a diagram of the differential sample and hold circuit 908and a differential amplifier 909 when the first column is in the samplephase and the second column is in the hold phase, with capacitors Cin2being connected to the input of the differential amplifier 909. FIG. 10Lshows a diagram of the differential sample and hold circuit 908 and adifferential amplifier 909 when the second column is in the sample phaseand the first column is in the hold phase, with capacitors Cin1 beingconnected to the input of the differential amplifier 909.

FIG. 10M shows readout circuitry shared by more than two columnsincluding a differential sample and hold circuit 908 and a differentialamplifier 909. FIG. 10M is similar to FIG. 10F in that a differentialamplifier 901 is shared by more than two columns, with the use of adifferential sample and hold circuit 908 and a differential amplifier909.

Dark Current Sampling

As understood by those of ordinary skill in the art, “dark current” iscurrent that is produced in a photodetector when no light is beingdetected by the photodetector. Designing a photodetector to correct forthe effect of dark current can improve the quality of photodetection.

In some embodiments of the integrated device described herein, one ormore of the charge storage bins may be used to sample the dark current.For example, a charge storage bin may sample dark current by aggregatingcarriers that arrive during a time period in which no light or a verylow level of light is received by the photodetector. In someembodiments, such as those relating to fluorescence lifetimemeasurements, the last bin (e.g., bin3) may be used to sample the darkcurrent if the timing is such that it occurs once the probability oflight emission drops to a negligible value. Sampling the dark currentmay allow subtracting the dark current from samples in other bins,thereby correcting for the effect of dark current.

Number and Timing of Time Bins

Any suitable number of time bins may be used. In FIGS. 3A and 3B, anexample of a pixel with four time bins has been illustrated. FIG. 8Cshows a plot in which eight bins are used. However, a pixel having anysuitable number of time bins may be produced based on the desiredtemporal resolution and other factors. Increasing the number of bins mayincrease the area taken up by each pixel, and may be achieved byreducing the overall number of pixels or by using a fabrication processhaving a smaller feature size. Using a small number of bins may allowincreasing the number of pixels that can fit on a chip. In someembodiments, a single bin may be used to determine the number of photonsarriving within a particular time period. The number of bins may beincreased or decreased at least in part by increasing or decreasing thenumber extensions of the charge carrier confinement region fabricated onthe chip extending from the carrier travel/capture region 106. Thenumber of electrodes b0-bm-1, transfer electrodes, etc., may beincreased or decreased accordingly based on the number of bins desiredto be included in a pixel.

The timing of the time bins may be chosen in any suitable way. In someembodiments, the timing may be selected by setting start and end timesfor the time bin(s), as illustrated in FIG. 6K. For example, the timingfor bin0 may be set by selecting the times at which t1 and t2 occur, andthe timing of the remaining bins may be set similarly.

In some embodiments, the timing for the time bins may be a fixed suchthat the timing is the same in each measurement period. The timing maybe set based upon a global timing signal. For example, a timing signalmay establish the start of a measurement period, and time bins may becontrolled to start and end based upon a predetermined amount of timehaving elapsed from the timing signal. In the fluorescence lifetimemeasurement context, the timing for the time bins may be set withrespect to the timing of an excitation pulse based upon the possiblerange of fluorescence lifetimes that are expected to be detected. In thetime-of-flight imaging context, the timing of the time bins may be setbased on an expected distance range for the scene to be imaged. However,in some embodiments the timing of the time bins may be variable orprogrammable.

In some embodiments, the timing for the time bins may be set based uponthe timing of a trigger event 702 that initiates a measurement periodfor a measurement 720. In the fluorescence lifetime measurement context,the timing for the time bins may be set in response to detecting thetiming of an excitation pulse that excites a fluorophore. For example,when an light excitation pulse reaches the pixel 100, a surge ofcarriers may travel from the photon absorption/carrier generation region102 to the drain 104. The accumulation of photogenerated carriers at thedrain 104 in response to the excitation pulse may cause a change involtage of the drain 104. Accordingly, in some embodiments theexcitation pulse may be detected by detecting the voltage of the drain104. For example, a comparator may compare the voltage of the drain 104to a threshold, and may produce a pulse when the voltage of the drain104 exceeds the threshold. The timing of the pulse may be indicate thetiming of the trigger event 702, and the timing of the time bins (e.g.,t1, t2, etc.) may be set based upon this timing. However, the techniquesdescribed herein are not limited in this respect, as any suitabletechnique may be used to detect the start of a measurement 720.

In some embodiments, the integrated device may be programmable to enablechanging the timing of the time bins. In some embodiments, the timing ofthe time bins may be programmed for a particular set of measurements tobe performed. For example, if the integrated device is used for a firsttype of test using a first set of markers having lifetimes within afirst range, the time bins may be programmed to suitable values fordiscriminating lifetimes of the markers within that range. However, ifthe integrated device is used for another type of test that usesdifferent markers having different lifetimes, the time bins may bechanged by programming them to correspond to different time intervalssuitable for the markers used in the second type of test.

In some embodiments, the timing of the time bins may be controlledadaptively between measurements based on the results of a set ofmeasurements. For example, as illustrated in FIG. 11, a first set ofmeasurements (Measurement Set A) may be performed using a first set oftime bins that span a relatively large time interval. The quantity ofphotons that arrived for each bin may be analyzed to determine whether achange should be made to the timing selected for the time bins toimprove or optimize the temporal information obtained. In someembodiments, the quantity of photons that arrive for each bin may beanalyzed to determine a narrower time interval of interest. For example,after performing a set of measurements with time bins as shown inMeasurement Set A of FIG. 11, it may be determined that a significantnumber of photons arrived in the time period corresponding to bin2 andno photons arrived in the time periods corresponding to other bins. Asecond set of time bins may then be selected for a second set ofmeasurements (Measurement Set B) that focuses on the narrower timeperiod corresponding to bin2 of Measurement Set A. As illustrated inFIG. 11, Measurement Set B has four time bins within the time periodcorresponding to bin2 of Measurement Set A. By performing measurementswith time bins according to Measurement Set B, further detail about thetiming of arrival of photons may be obtained. For example, asillustrated in FIG. 11, higher temporal resolution about the timing ofarrival of incident photons may be obtained within a selected timeinterval. Such an adaptive time bin determination process may allowobtaining a level of time resolution using a relatively small number ofbins (e.g., 4 bins) that otherwise may necessitate a large number ofbins (e.g., 16 bins).

In some embodiments, the timing for the time bins may be the same in allpixels of the array. In some embodiments, the timing may be different indifferent pixels such that different pixels capture carriers indifferent time bins. For example, a first set of pixels may capturecarriers in a first set of time bins, and a second set of pixels maycapture carriers in a second set of time bins that are at leastpartially different from the first set of time bins. For example, onerow of pixels may have the time timing for their time bins and anotherrow of pixels may have a different timing for their time bins. In someembodiments, a first set of rows of pixels (e.g., four rows) may havethe same timing for their time bins, and another set of rows of pixels(e.g., another four rows) may have a different timing for their timebins. Pixels may be set and/or programmed individually and/or as agroup.

Pixels with Sub-Pixels

Wavelength Discrimination

In some embodiments, a pixel of a pixel array may include a plurality ofsub-pixels that are each capable of performing different types ofmeasurements. Any number of sub-pixels may be included in a pixel.

FIG. 12 shows an example of a pixel 1100 that includes four sub-pixels100A. In some embodiments, each sub-pixel 100A in pixel 1100 may beconfigured to receive light of a different wavelength. For example,filters may be formed above sub-pixels 100A that allow photons ofdifferent wavelengths to be transmitted to sub-pixels 100A. For example,a first wavelength may be transmitted to a first sub-pixel 100A, asecond wavelength may be transmitted to a second sub-pixel 100A, a thirdwavelength may be transmitted to a third sub-pixel 100A, and a fourthwavelength may be transmitted to a fourth sub-pixel 100A. A pixel 1100having sub-pixels configured to receive light of different wavelengthsmay allow both temporal and spectral discrimination of incident light.In the fluorescence lifetime measurement context, providing thecapability of both temporal and spectral discrimination may allowdiscriminating markers having different lifetimes, different spectralcharacteristics, or markers having both different lifetimes anddifferent spectral characteristics.

Temporal Discrimination

In some embodiments, different sub-pixels 100A may be controlled tosample time bins for different time intervals. For example, a firstsub-pixel 100A may be configured to sample a first set of time bins anda second sub-pixel may be configured to sample a second set of timebins. Similar structures in different sub-pixels 100A may sample timebins for different time intervals by controlling the timing of thecharge carrier segregation structure to be different in differentsub-pixels.

Pixel Array/Chip Architecture

FIG. 13 shows a diagram of the chip architecture, according to someembodiments. As shown in FIG. 13, an integrated circuit or chip 1300 mayinclude a pixel array 1302 including a plurality of pixels 100, acontrol circuit 1304 that includes a timing circuit 1306,voltage/current bias generation circuits 1305 and an interface 1308.

Pixel array 1302 includes an array of pixels 101 laid out in anysuitable pattern, such as a rectangular pattern, for example. The pixelarray 1302 may have any suitable number of pixels. In some embodiments,the pixel array may have a 64×64 array of 4096 pixels 101, eachincluding four sub-pixels 101A. However, the techniques described hereinare not limited as to the number or arrangement of pixels and sub-pixelsincluded in the pixel array 1302. The pixel array may have row and/orcolumn conductors for reading out rows or columns of the pixel array1302. Pixels may be read out in parallel, in series, or a combinationthereof. For example, in some embodiments a row of pixels may be readout in parallel, and each row of the pixel array may be read outsequentially. However, the techniques described herein are not limitedin this respect, as the pixels may be read out in any suitable manner.

The pixel array 1302 is controlled by a control circuit 1304. Controlcircuit 1304 may be any suitable type of control circuit for controllingoperations on the chip 1300, including operations of the pixel array1302. In some embodiments, control circuit 1304 may include amicroprocessor programmed to control operations of the pixel array 1302and any other operations on the chip 1300. The control circuit mayinclude a computer readable medium (e.g., memory) storing computerreadable instructions (e.g., code) for causing the microprocessorperforming such operations. For example, the control circuit 1304 maycontrol producing voltages to be applied to electrodes of the chargecarrier segregation structure(s) in each pixel. The control circuit 1304may change the voltages of one or more electrodes, as discussed above,to capture carriers, transfer carriers, and to perform readout of pixelsand the array. The control circuit may set the timing of operations ofthe charge carrier segregation structure based on a stored timingscheme. The stored timing scheme may be fixed, programmable and/oradaptive, as discussed above.

The control circuit 1304 may include a timing circuit 1306 for timingoperations of the charge carrier segregation structure(s) of the pixelsor other operations of the chip. In some embodiments, timing circuit1306 may enable producing signals to precisely control the timing ofvoltage changes in the charge carrier segregation structure(s) toaccurately time bin charge carriers. In some embodiments the timingcircuit 1306 may include an external reference clock and/or adelay-locked loop (DLL) for precisely setting the timing of the signalsprovided to the charge carrier segregation structure(s). In someembodiments, two single-ended delay lines may be used, each with halfthe number of stages aligned 180-degrees out of phase. However, anysuitable technique may be used for controlling the timing of signals onthe chip.

The chip 1300 may include an interface 1308 for sending signals from thechip 1300, receiving signals at the chip 1300, or both. The interface1308 may enable reading out the signals sensed by the pixel array 1302.Readout from the chip 1300 may be performed using an analog interfaceand/or a digital interface. If readout from the chip 1300 is performedusing a digital interface, the chip 1300 may have one or more analog todigital converters for converting signals read out from the pixel array1302 into digital signals. In some embodiments, the readout circuit mayinclude a Programmable Gain Amplifier. One or more control signals maybe provided to the chip 1300 from an external source via interface 1308.For example, such control signals may control the type of measurementsto be performed, which may include setting the timing of the time bins.

Analysis of signals read out from the pixel array 1302 may be performedby circuitry on-chip or off-chip. For example, in the context offluorescence lifetime measurement, analysis of the timing of photonarrival may include approximating a fluorescence lifetime of afluorophore. Any suitable type of analysis may be performed. If analysisof signals read out from the pixel array 1302 is performed on-chip, chip1300 may have any suitable processing circuitry for performing theanalysis. For example, chip 1300 may have a microprocessor forperforming analysis that is part of or separate from control circuit1304. If analysis is performed on-chip, in some embodiments the resultof the analysis may be sent to an external device or otherwise providedoff-chip through interface 1308. In some embodiments all or a portion ofthe analysis may be performed off-chip. If analysis is performedoff-chip, the signals read out from the pixel array 1302 and/or theresult of any analysis performed by the chip 1300, may be provided to anexternal device through interface 1308.

In some embodiments, the chip 1300 may include one or more of thefollowing:

-   -   1) on-chip, digitally controlled, pixel bias generators (DACs).    -   2) on-chip, digitally programmable gain amplifiers that convert        the single-ended pixel output voltage signal to a differential        signal and applies gain to the signal    -   3) digitally-controlled amplifier bias generators that allow        scaling the power dissipation with the output rate.

FIG. 14A shows a diagram of an embodiment of a chip 1300A, which is anexample of chip 1300 having a 64×64 array of quad pixels, according tosome embodiments. In the embodiment of FIG. 14A, half of the pixeloutput signals are provided via the top side of the chip and the otherhalf of the pixel output signals are provided via the bottom side of thechip. Bias circuits are included for setting the voltage of theelectrodes of the charge carrier segregation structures.

FIG. 14B shows a diagram of an embodiment of a chip 1300B, which is anexample of chip 1300 includes 2×2 arrays, with each array having 256×64octal pixels array of quad pixels, according to some embodiments.Bandgap and bias circuits are included. Digital to analog converts(DACs), including Vhigh DACs and Vlow DACs are included for setting thehigh and low voltages of the electrodes of the pixel array. FIG. 14Balso shows light monitoring sensors 1320. Each light monitoring sensormay include a photodetector, such as a photodiode. In some embodiments,each light monitoring sensor may include a quad array of photodetectors(e.g., photodiodes) for aligning the chip 1300B with a light source. Inan embodiment in which the chip 1300B is configured for detection ofmolecules, the light monitoring sensors may enable alignment of the chip1300B with a waveguide that receives light from one or more locations inwhich the molecules are positioned. Diode readout circuits and a diodeselect register is also shown in FIG. 14B.

Examples of array sizes, dimensions, numbers of bins, and feature sizesare described above and shown in the figures merely by way ofillustration, as any suitable of array sizes, dimensions, numbers ofbins, and feature sizes may be used.

Example Integrated Circuit Realization and Method of Forming theIntegrated Photodetector

In some embodiments, the chip 1300 may be formed in a silicon substrateusing a standard CMOS (Complementary Metal Oxide Semiconductor) process.However, the techniques described herein are not limited in thisrespect, as any suitable substrate or fabrication process may be used.

FIGS. 15-22 illustrate a process of forming a chip 1300, according tosome embodiments.

FIG. 15A shows a perspective view of charge confinement regions 103 thatmay be formed in a semiconductor substrate. FIG. 15B shows a plan viewcorresponding to FIG. 15A. In some embodiments, charge confinementregions 103 may be formed in a bulk semiconductor substrate 1500.However, the techniques described herein are not limited to use of abulk semiconductor substrate, as any suitable type of semiconductorsubstrate may be used. In some embodiments, the substrate 1500 andcharge confinement regions 103 may be formed of monocrystalline silicon.However, the techniques described herein are not limited in thisrespect, as any suitable type of semiconductor material may be used. Insome embodiments, using a silicon substrate may enable using acost-effective industry standard CMOS process. However, any suitablefabrication process may be used. In some embodiments, a bulk siliconsubstrate having a p-type doping type may be used. However, any suitabledoping type may be used, including n-type doping or p-type doping.

As shown in FIG. 15A, the charge confinement regions 103 may be a raisedportion of substrate 1500. Charge confinement regions 103 may be formedby etching away regions of the substrate 1500 in the pattern shown inFIGS. 15A and 15B, thereby leaving raised charge confinement regions 103extending above the substrate. An insulating layer may then be formedover and to the side of the charge confinement regions 103. For example,in some embodiments an insulating layer of silicon oxide may be formedon charge confinement regions 103 by thermal growth. However, anysuitable technique may be used to form the insulating layer, and theinsulating layer may include any suitable insulating material.

As shown in FIG. 16, electrodes as illustrated in FIG. 3B may be formedover the insulating layer by forming a patterned polysilicon layer 1601.The electrodes may be spaced apart from one another to allow differentelectrodes to be at different voltages. The electrodes may be formed ofany suitable conductive material. In some embodiments, the electrodesmay be formed of doped polysilicon. However, the techniques describedherein are not limited to forming the electrodes of polysilicon, as anysuitable conductive material may be used to form the electrodes (e.g., ametal). Conductive vias 1701 may be formed over the patternedpolysilicon layer 1601 to contact the polysilicon layer 1601 through aninsulating layer (not shown) overlying the patterned polysilicon layer1601. The conductive vias 1701 may be formed of any suitable conductor.

In some embodiments, one or more electrodes (e.g., of polysilicon layer1601) may be split-doped electrodes having both p− and n− type dopants.A split-doped electrode may enable forming a potential well to capture acarrier, as illustrated in FIG. 17. FIG. 17 shows a split-dopedelectrode 2302 having a p+ region and an n+ region. The n+ region andthe p+ region produce different potential levels in the underlyingsemiconductor. As shown in FIG. 17, the n+ region of split-dopedelectrode 2302 may produce a potential well under the n+ region that canconfine charge carriers (e.g., electrons). FIG. 17 illustrates thatkeeping the voltage of the split-doped electrode 2302 high may produce apotential gradient as shown in dashed lines, which may confine chargecarriers (e.g., electrons) in a potential well 2304. Lowering thevoltage of split-doped electrode 2302 may raise the electric potentialunder the split-doped electrode 2302 to enable transferring chargetrapped in the potential well 2304 to a charge storage bin, for example.

Dopants may be formed in the semiconductor material to enable formingtransistors of the readout circuitry 110. In some embodiments, a maskmay be disposed over the charge confinement region 103 to prevent thedoping of charge confinement region 103 during the formation of thetransistors of readout circuitry 110, as doping charge confinementregion 103 may form undesired potential wells in the charge confinementregions 103.

FIG. 18 shows the formation of a metal layer 1801 (e.g., metal 1) overthe patterned polysilicon layer 1601 to connect to the vias 1701. FIG.19 shows the metal layer 1801 overlaid on the polysilicon layer 1601 andcharge confinement regions 103.

FIG. 20 shows the formation of vias 1901 to contact the metal layer1801. Conductive vias 1901 may be formed over the metal layer 1801 tocontact the metal layer 1801 through an insulating layer (not shown)overlying the metal layer 1801. FIG. 20 also shows the formation of asecond metal layer 2001 (e.g., metal 2) over the metal layer 1801 andvias 1901.

FIG. 21 shows the second metal layer 2001 as well as formation of via(s)2101 over the metal layer 2001 to contact the metal layer 2001 throughan insulating layer (not shown) overlying the metal layer 2001.

FIG. 22 shows the formation of a third metal layer 2201 (e.g., metal 3)over the metal layer 2001 and the via(s) 2101 to contact the vias 2101.

The foregoing process is described by way of illustration, as thetechniques described here are not limited to any particular fabricationprocess. Further, the techniques described herein are not limited as tothe particular layout shown.

Drive Circuitry for the Charge Carrier Segregation Structure

The electrodes of the charge carrier segregation structure that overliethe substrate may have a substantial parasitic capacitance. Changing thevoltages on the electrodes necessitates charging or discharging theparasitic capacitance. The speed with which current can be provided tocharge or discharge the parasitic capacitance limits the speed at whichthe voltage of an electrode can be changed. As discussed above, in someembodiments charge carriers may be captured and transferred into timebins with nanosecond or picosecond resolution. The inventors haverecognized and appreciated that the timing with which charge carriersmay be captured may have a higher precision if the voltage of electrodesb0-bm-1 change more quickly, thereby raising the potential barriers atprecise moments in time. However, rate of change of the voltage onelectrodes b0-bm-1 is limited due to the parasitic inductance andequivalent series resistance (ESR) of the connection between the voltagesupply and the electrodes b0-bm-1.

Further, charging and discharging the parasitic capacitances of theelectrodes may consume significant power. The power dissipated bycharging and discharging an electrode is P_(diss)=(½)·f·C—V², where C isthe capacitance between the electrode and the substrate, V is thevoltage difference between the electrode and the substrate, and f is thefrequency with which the voltage is switched.

FIG. 23 shows an example of a drive circuit 2300 for driving anelectrode 2301 of the charge carrier segregation structure, according tosome embodiments. Electrode 2301 is illustrated as a capacitor in FIG.23. As discussed above, the electrode 2301 may be driven to a relativelylow voltage V_(low) and a relatively high voltage V_(high) at selectedtimes. The drive circuit 2300 includes a VdacH generator 2302 thatproduces the high voltage V_(high) and a VdacL generator 2304 thatproduces the low voltage V_(low). In some embodiments, the differencebetween V_(low) and V_(high), may be made as small as possible for theelectrode to influence charge carriers in the manner designed, therebyreducing or minimizing power dissipation. In some embodiments, VdacHgenerator 2302 and/or VdacL generator 2304 may be programmable voltagegenerators that can produce desired voltages V_(low) and/or V_(high),and can allow changing V_(low) and/or V_(high).

The drive circuit 2300 also includes Bclk generator 2306, which canproduce a timing signal for timing voltage transitions of the electrode2301. The Bclk generator 2306 may be programmable, and may allowdigitally selecting the times at which the edges of the timing signaloccur, based on an input digital word. In some embodiments, the Bclkgenerator 2306 may be implemented using a delay locked loop (DLL), asdiscussed above. The timing signal from the Bclk generator 2306 isprovided to the input of the Bclk driver 2312 which drives the electrode2301.

The drive circuit 2300 also includes a VdacH amplifier 2308 and a VdacLamplifier 2310. The VdacH amplifier 2308 receives a signal from theVdacH generator and controls transistor 2314 using feedback to providethe voltage VdacH to the high power supply terminal of the Bclk driver2312. The VdacH amplifier 2308 also charges capacitor 1312A to thevoltage VdacH. The VdacL amplifier 2310 receives a signal from the VdacLgenerator and controls transistor 2316 using feedback to provide thevoltage VdacL to the low power supply terminal of the Bclk driver 2312.The VdacL amplifier 2310 also charges capacitor 1312B to the voltageVdacL.

As discussed above, the electrode 2301 may have substantial capacitance.To supply enough current to charge the electrode 2301 with high speed,decoupling capacitors 1312A and 1312B may be provided to supply currentto the to the low power supply terminal of the Bclk driver 2312 or thehigh power supply terminal of the Bclk driver 2312 during transitions.

The decoupling capacitor(s) may be positioned in close proximity to theelectrode to limit the parasitic inductance and equivalent seriesresistance (ESR) between the electrode and the decoupling capacitor.When the voltage of an electrode is changed to a new voltage, theelectrode is connected to the decoupling capacitor at the new voltage tosupply current to the electrode through a current path having lowparasitic inductance and/or equivalent series resistance (ESR), so thatthe voltage of the electrode can be changed quickly. In someembodiments, the decoupling capacitor may be positioned close enough tothe electrode such that the parasitic inductance between the decouplingcapacitor and the electrode is less than 3 nH, less than 2 nH, or lessthan 1 nH. In some embodiments, the equivalent series resistance (ESR)of the current path between the decoupling capacitor and the electrodeis less than 70 ohms, less than 35 ohms, or less than 5 ohms. However,these values are provided merely by way of example, as the techniquesdescribed herein are not limited to specific values of inductance orresistance.

In some embodiments, electrodes b0-bm-1 may be connectable to one ormore decoupling capacitors. In some embodiments, each electrode b0-bm-1may have its own decoupling capacitors(s). For example, in someembodiments an electrode may have a single decoupling capacitor coupledbetween the high and low voltage supplies of the electrode, or twodecoupling capacitors respectively coupled to the high voltage supplyand the low voltage supply. However, the techniques described herein arenot limited in this respect. Any or all of the electrodes of the chargecarrier segregation structure may be connected to decoupling capacitors.

The decoupling capacitors may have any suitable capacitance value. Insome embodiments, the capacitance value of a decoupling capacitor is tento one hundred times the capacitance of the electrode to which it is tobe connected. In some embodiments, the capacitance of a decouplingcapacitor may be at least 150 pF, at least 300 pF, or at least 3 nF orhigher. However, these values are provided merely by way of example, asthe techniques described herein are not limited to specific values ofcapacitance.

A decoupling capacitor may be on-chip or off-chip. FIG. 24 shows anembodiment in which chip 1300 is affixed to a printed circuit board1310, which may be termed a “chip-on-board” or “die-on-board”implementation. Wire bonds may connect the chip 1300 to one or moredecoupling capacitors 1312 on the printed circuit board 1310, therebyproviding current path having low parasitic inductance and/or equivalentseries resistance (ESR) between an electrode of the chip 1300 and adecoupling capacitor 1312. In some embodiments, off-chip decouplingcapacitors may be positioned within 1 cm, or within 5 mm of the chip1300 or less. However the techniques described herein are not limited inthis respect. As mentioned above, decoupling capacitor(s) may be formedon the chip 1300.

As discussed above, charging and discharging the electrodes of thecharge carrier segregation structure may dissipate significant power. Insome embodiments, the one or more rows of pixels of the chip 1300 andtheir corresponding electrodes may be disabled, which may limit thepower consumption of the chip 1300. The chip 1300 may be programmable inthis respect, and may allow selecting which rows will be enabled ordisabled. The rows that are enabled and disabled may be changed overtime.

FIG. 25 illustrates enabling 32 rows in a central region of the chip anddisabling 48 rows at the edges of the chip. Disabling one or more rowsof the chip may allow reducing power consumption in situations orapplications where not all the rows of the chip are needed.

ADDITIONAL ASPECTS

In some embodiments, techniques described herein may be carried outusing one or more computing devices. Embodiments are not limited tooperating with any particular type of computing device.

FIG. 26 is a block diagram of an illustrative computing device 1000 thatmay be used to implement a control circuit for controlling the pixelarray or for performing analysis of the data from the pixels. Computingdevice 1000 may include one or more processors 1001 and one or moretangible, non-transitory computer-readable storage media (e.g., memory1003). Memory 1003 may store, in a tangible non-transitorycomputer-recordable medium, computer program instructions that, whenexecuted, implement any of the above-described functionality.Processor(s) 1001 may be coupled to memory 1003 and may execute suchcomputer program instructions to cause the functionality to be realizedand performed.

Computing device 1000 may also include a network input/output (I/O)interface 1005 via which the computing device may communicate with othercomputing devices (e.g., over a network), and may also include one ormore user I/O interfaces 1007, via which the computing device mayprovide output to and receive input from a user. The user I/O interfacesmay include devices such as a keyboard, a mouse, a microphone, a displaydevice (e.g., a monitor or touch screen), speakers, a camera, and/orvarious other types of I/O devices.

The above-described embodiments can be implemented in any of numerousways. For example, the embodiments may be implemented using hardware,software or a combination thereof. When implemented in software, thesoftware code can be executed on any suitable processor (e.g., amicroprocessor) or collection of processors, whether provided in asingle computing device or distributed among multiple computing devices.It should be appreciated that any component or collection of componentsthat perform the functions described above can be generically consideredas one or more controllers that control the above-discussed functions.The one or more controllers can be implemented in numerous ways, such aswith dedicated hardware, or with general purpose hardware (e.g., one ormore processors) that is programmed using microcode or software toperform the functions recited above.

In this respect, it should be appreciated that one implementation of theembodiments described herein comprises at least one computer-readablestorage medium (e.g., RAM, ROM, EEPROM, flash memory or other memorytechnology, CD-ROM, digital versatile disks (DVD) or other optical diskstorage, magnetic cassettes, magnetic tape, magnetic disk storage orother magnetic storage devices, or other tangible, non-transitorycomputer-readable storage medium) encoded with a computer program (i.e.,a plurality of executable instructions) that, when executed on one ormore processors, performs the above-discussed functions of one or moreembodiments. The computer-readable medium may be transportable such thatthe program stored thereon can be loaded onto any computing device toimplement aspects of the techniques discussed herein. In addition, itshould be appreciated that the reference to a computer program which,when executed, performs any of the above-discussed functions, is notlimited to an application program running on a host computer. Rather,the terms computer program and software are used herein in a genericsense to reference any type of computer code (e.g., applicationsoftware, firmware, microcode, or any other form of computerinstruction) that can be employed to program one or more processors toimplement aspects of the techniques discussed herein.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is: 1-20. (canceled)
 21. An integrated circuit,comprising: a photodetection region; a first charge carrier storageregion; a second charge carrier storage region; and a floating diffusionregion, wherein the integrated circuit is configured to: receive, at thephotodetection region in response to an excitation light pulse, aplurality of excitation photons; discard a plurality of charge carriersgenerated in the photodetection region in response to receiving theplurality of excitation photons in the photodetection region; receive,at the photodetection region after receiving the plurality of excitationphotons, a first plurality of fluorescence photons; transfer, from thephotodetection region to the first charge carrier storage region, afirst plurality of fluorescence charge carriers generated in thephotodetection region in response to receiving the first plurality offluorescence photons; receive, at the photodetection region afterreceiving the first plurality of fluorescence photons, a secondplurality of fluorescence photons; transfer, from the photodetectionregion to the second charge carrier storage region, a second pluralityof fluorescence charge carriers generated in the photodetection regionin response to receiving the second plurality of fluorescence photons;transfer, from the first charge carrier storage region to the floatingdiffusion region, a first number of charge carriers stored in the firstcharge carrier storage region; sample, from the floating diffusionregion, a first voltage corresponding to the first number of chargecarriers; transfer, from the second charge carrier storage region to thefloating diffusion region, a second number of charge carriers stored inthe second charge carrier storage region; and sample, from the floatingdiffusion region, a second voltage corresponding to a second number ofcharge carriers stored in the second charge carrier storage region. 22.The integrated circuit of claim 21, wherein the photodetection region isconfigured to induce an electric field that produces a force in adirection along which the photodetection region is configured to receiveincident photons from a light source.
 23. The integrated circuit ofclaim 21, wherein the integrated circuit is configured to induce anelectric field that produces a force in a direction from thephotodetection region toward the first and/or second charge carrierstorage region.
 24. The integrated circuit of claim 23, wherein thephotodetection region comprises a graded doping concentration configuredto induce the electric field that produces the force in the directionfrom the photodetection region toward the first and/or second chargecarrier storage region.
 25. The integrated circuit of claim 21, whereinthe integrated circuit is further configured to: sample a voltage of thefloating diffusion region before transferring the first number of chargecarriers to the floating diffusion region; subtract, from the firstvoltage corresponding to the first number of charge carriers, thevoltage of the floating diffusion region sampled before transferring thefirst number of charge carriers to the floating diffusion region; samplea voltage of the floating diffusion region before transferring thesecond number of charge carriers to the floating diffusion region; andsubtract, from the second voltage corresponding to the second number ofcharge carriers, the voltage of the floating diffusion region sampledbefore transferring the second number of charge carriers to the floatingdiffusion region.
 26. The integrated circuit of claim 21, wherein theintegrated circuit is further configured to: sample a voltage of thefloating diffusion region before transferring the first number of chargecarriers to the floating diffusion region; subtract, from the firstvoltage corresponding to the first number of charge carriers, thevoltage of the floating diffusion region sampled before transferring thefirst number of charge carriers to the floating diffusion region; andsubtract, from the second voltage corresponding to the second number ofcharge carriers, the first voltage corresponding to the first number ofcharge carriers, wherein the second voltage corresponds to a combinednumber of charge carriers including the first number of charge carriersand the second number of charge carriers.
 27. The integrated circuit ofclaim 21, wherein the integrated circuit is configured to aggregate thefirst number of charge carriers in the first charge carrier storageregion and the second number of charge carriers in the second chargecarrier storage region over a plurality of measurements performed inresponse to a plurality of excitation pulses, respectively.
 28. Theintegrated circuit of claim 27, wherein: the first number of the firstplurality of charge carriers correspond to fluorescence photons receivedat the photodetection region during a first time period after anexcitation light pulse; and the second number of the second plurality ofcharge carriers correspond to fluorescence photons received at thephotodetection region during a second time period after an excitationlight pulse.
 29. The integrated circuit of claim 21, further comprising:a first electrode configured to, when a first voltage is applied to thefirst electrode, induce an electric field that produces a first force ina direction from the photodetection region to the first charge carrierstorage region to transfer the first plurality of fluorescence chargecarriers from the photodetection region to the first charge carrierstorage region; and a second electrode configured to, when a secondvoltage is applied to the second electrode, induce an electric fieldthat produces a second force in a direction from the photodetectionregion to the second charge carrier storage region to transfer thesecond plurality of fluorescence charge carriers from the photodetectionregion to the second charge carrier storage region.
 30. The integratedcircuit of claim 21, wherein the integrated circuit comprises aplurality of pixels, and the integrated circuit is configured to: samplethe first and second voltages from a first pixel; and after sampling thefirst and second voltages from the first pixel, sample a third voltagecorresponding to a third number of charge carriers stored in a firstcharge carrier storage region of a second pixel; and after sampling thethird voltage, sample a fourth voltage corresponding to a fourth numberof charge carriers stored in a second charge carrier storage region ofthe second pixel.
 31. A method, comprising: receiving, at aphotodetection region of an integrated circuit in response to anexcitation light pulse, a plurality of excitation photons; discarding aplurality of charge carriers generated in the photodetection region inresponse to receiving the plurality of excitation photons in thephotodetection region; receiving, at the photodetection region afterreceiving the plurality of excitation photons, a first plurality offluorescence photons; transferring, from the photodetection region to afirst charge carrier storage region of the integrated circuit, a firstplurality of fluorescence charge carriers generated in thephotodetection region in response to receiving the first plurality offluorescence photons; receiving, at the photodetection region afterreceiving the first plurality of fluorescence photons, a secondplurality of fluorescence photons; transferring, from the photodetectionregion to a second charge carrier storage region of the integratedcircuit, a second plurality of fluorescence charge carriers generated inthe photodetection region in response to receiving the second pluralityof fluorescence photons; transferring, from the first charge carrierstorage region to a floating diffusion region of the integrated circuit,a first number of charge carriers stored in the first charge carrierstorage region; sampling, from the floating diffusion region, a firstvoltage corresponding to the first number of charge carriers;transferring, from the second charge carrier storage region to thefloating diffusion region, a second number of charge carriers stored inthe second charge carrier storage region; and sampling, from thefloating diffusion region, a second voltage corresponding to a secondnumber of charge carriers stored in the second charge carrier storageregion.
 32. The method of claim 31, further comprising inducing, in thephotodetection region, an electric field that produces a force in afirst direction, wherein the photodetection region is configured toreceive incident photons along the first direction.
 33. The method ofclaim 31, further comprising inducing an electric field that produces aforce from the photodetection region toward the first and/or secondcharge carrier storage regions.
 34. The method of claim 33, wherein theelectric field is induced, at least in part, by a graded dopingconcentration of the photodetection region.
 35. The method of claim 31,further comprising: sampling a voltage of the floating diffusion regionbefore transferring the first number of charge carriers to the floatingdiffusion region; subtracting, from the first voltage corresponding tothe first number of charge carriers, the voltage of the floatingdiffusion region sampled before transferring the first number of chargecarriers to the floating diffusion region; sampling a voltage of thefloating diffusion region before transferring the second number ofcharge carriers to the floating diffusion region; and subtracting, fromthe second voltage corresponding to the second number of chargecarriers, the voltage of the floating diffusion region sampled beforetransferring the second number of charge carriers to the floatingdiffusion region.
 36. The method of claim 31, further comprising:sampling a voltage of the floating diffusion region before transferringthe first number of charge carriers to the floating diffusion region;subtracting, from the first voltage corresponding to the first number ofcharge carriers, the voltage of the floating diffusion region sampledbefore transferring the first number of charge carriers to the floatingdiffusion region; and subtracting, from the second voltage correspondingto the second number of charge carriers, the first voltage correspondingto the first number of charge carriers, wherein the second voltagecorresponds to a combined number of charge carriers including the firstnumber of charge carriers and the second number of charge carriers. 37.The method of claim 31, wherein the first number of charge carriersstored in the first charge carrier storage region and the second numberof charge carriers stored in the second charge carrier storage regionare each aggregated over a plurality of measurements performed inresponse to a plurality of excitation pulses, respectively.
 38. Themethod of claim 37, wherein: the first number of the first plurality ofcharge carriers correspond to fluorescence photons received at thephotodetection region during a first time period after an excitationlight pulse; and the second number of the second plurality of chargecarriers correspond to fluorescence photons received at thephotodetection region during a second time period after an excitationlight pulse.
 39. The method of claim 31, wherein: transferring the firstplurality of fluorescence charge carriers from the photodetection regionto the first charge carrier storage region comprises applying a firstvoltage to a first electrode to induce an electric field that produces afirst force in a direction from the photodetection region to the firstcharge carrier storage region; and transferring the second plurality offluorescence charge carriers from the photodetection region to thesecond charge carrier storage region comprises applying a second voltageto a second electrode to induce an electric field that produces a secondforce in a direction from the photodetection region to the second chargecarrier storage region.
 40. The method of claim 31, wherein the firstand second voltages are sampled from a first pixel of a plurality ofpixels of the integrated circuit, and wherein the method furthercomprises: after sampling the first and second voltages from the firstpixel, sampling a third voltage corresponding to a third number ofcharge carriers stored in a first charge carrier storage region of asecond pixel; and after sampling the third voltage, sampling a fourthvoltage corresponding to a fourth number of charge carriers stored in asecond charge carrier storage region of the second pixel.